Why are Hydrogrossular Stones Called Garnets?

These four photos are of hydrogrossular garnets discovered on Gemstone Beach, near Orepuki (the last two photos are of the two sides of the same stone). They don’t look like garnets, do they? We are used seeing garnets like this:


The word “garnet” comes from the 14th‑century Middle English word “gernet”, meaning “dark red”. It is thought to be derived from the Latin “granatus”, from “granum” (meaning “grain, seed”), and possibly a reference to “pomum granatum”, the pomegranate, a plant whose fruits contain abundant and vivid red seed covers which are similar in shape, size, and color to some garnet crystals. In fact, garnets can be found in a wide range of colours, although red is the most common.

In a previous Post, What are Hydrogrossular Garnets?, it was noted that the chemical composition of hydrogrossular garnets is hydrous calcium aluminium silicate and that hydrogrossular garnets hardly ever occur as good crystals but rather as dense masses. This Post aims to explain why hydrogrossular garnets are called garnets and why they are not crystals.

In the section on “Garnet” in Geology.com, it is noted that Garnet is the name used for a large group of rock-forming minerals: These minerals share a common crystal structure and a generalized chemical composition of X3Y2(SiO4)3. In that composition, “X” can be Ca, Mg, Fe2+ or Mn2+, and “Y” can be Al, Fe3+, Mn3+, V3+ or Cr3+.

Click on the Table below to see how different types of garnets have different chemical compositions in which the X and Y position for chemical elements vary: 

The Note on this Table (above) points out that “the compositions listed…are for end members of several solid solution series”. In other words, there are gradations in the series of stones we call garnets and when we group them we are breaking up a coninuum, creating “end members” at the start and end of each group. There is actually a wide range of types of garnets that are grouped in various ways. Wikigempedia reports that the garnet family is divided into two sub-groups, Pyralspite which is Calcium-free garnet and Ugrandite which is Calcium-rich garnet. Almandine (iron-aluminium silicate), Pyrope (magnesium-aluminium silicate), and Spessartine (manganese-aluminium silicate) are the main members of Pyralspite sub-group, and Grossular (Calcium-aluminium silicate), Andradite (calcium-iron silicate) and, Uvarovite (calcium-chromium silicate) are the members of Ugrandite sub-group.

The excellent article on Garnet in Wikigempedia presents a table which summarises the common properties of all garnets as well as the variations in chemical formulae for three types of garnets: 

We are now able to make a little more sense of the Wikipedia entry on hydrogrossular, mentioned in the previous Post, where it states:

Hydrogrossular is a calcium aluminium garnet series (formula: Ca3Al2(SiO4)3−x(OH)4x, with hydroxide (OH) partially replacing silica (SiO4)). The endmembers of the hydrogarnet family (grossular, hibschite, and katoite) depend on the degree of substitution (x):
grossular: x = 0
hibschite: 0.2 < x < 1.5
katoite: 1.5 < x < 3.

The latter part of this statement is in effect saying that there are slightly different types of hydrogrossular caused by varying degrees of replacement of silica by hydroxide in the stone.  

The Geology.com article points out that garnets in general have a “vitreous [glassy] luster, a transparent-to-translucent diaphaneity [transparency], a brittle tenacity, and a lack of cleavage.” Furthermore, they can be found as individual crystals, stream-worn or beach-worn pebbles, granular aggregates, and/or massive occurrences (“massive” meaning lacking internal crystalline structure).

Wikipedia explains that hydrogrossular is “found in massive crystal habit, sometimes grown in with idocrase [another mineral]”. In terms of its transparency, hydrogrossular is “translucent to opaque”, and can be found in the colours of green to bluish green, pink, white, and gray. The cause of the green color is chromium, and possibly iron. Pink hydrogrossular is caused by the presence of manganese. Hydrogrossular may also have dark gray to black small inclusions.

Wikipedia mentions that hydrogrossular garnet has similarities to jade. This can perhaps be seen in the way in which it can be carved – see Russell Beck’s carvings in Te Ara The Encyclopedia of New Zealand.

The next Post in this series will consider why hydrogrossular garnets can primarily be found in two main regions in New Zealand, the Nelson area and the south-west of the South Island, some hundreds of kilometres apart.


What are Hydrogrossular Garnets?

I first came across reference to hydrogrossular garnets when gathering information online about the stones that could be found on Gemstone Beach, Orepuki, on the south coast of New Zealand’s South Island. Some brief descriptions of Gemstone Beach include comments along the following lines:  “Semi-precious gems such as garnet, jasper, quartz and nephrite can often be found on the beach. A few hours beachcombing could easily yield gems such as hydrogrossular, jasper, fossil worm casts and the elusive sapphire” (Source: Information about the Riverton–Aparima South Coast Heritage Trail. See also Nature’s Edge: Tuatapere and the mindat.org entry on Gemstone Beach.) The source of this could be a Heritage Trail sign that, to my knowledge, is no longer at Gemstone Beach (I don’t recall having seen it on any of my visits).

Gemstone Beach Heritage Trail
Source: https://talltales.me/2013/02/24/the-south-of-the-south/photo-2-01-13-10-30-21-am

The Encyclopedia of New Zealand mentions hydrogrossular garnets in the following terms:

Calcium-rich garnet is called grossular. A red form, found in South Westland, is known as hessonite. Another variety, containing some water, is called hydrogrossular and was first identified at the Roding River near Nelson. It is also found on the beach near Orepuki in Southland. Rounded lumps of pale green hydrogrossular take a good polish and have been used for jewellery.   Hydrogrossular pebbles, being heavy and exceptionally hard, were used by Southland Māori as hammer stones for the making of stone implements

In many ways, all this information raised more questions for me than it answered. What does a hydrogrossular garnet look like? Is is a valuable gemstone, like other garnets? Why is it a garnet? How often can these stones be found on Gemstone Beach?

For a while, I mistakenly referred to them as “hydroglossular” (“…gloss…” not “…gross…”), thinking that because they were likely to be shiny they would be glossy. On my first few trips to Gemstone Beach, I decided that there was a particular stone that was probably a “hydroglossular”, even thought it seemed quite dull. It was a grey stone that was kind of a dull quartz-like thing, looking like there was water within its fabric (the “hydro” part).

I collected some and tried polishing them but they were very unremarkable stones.

So what does “hydrogrossular” mean? Wikipedia gives an answer that refers to the physical-chemical make-up of the stone: 

Hydrogrossular is a calcium aluminium garnet series (formula: Ca3Al2(SiO4)3−x(OH)4x, with hydroxide (OH) partially replacing silica (SiO4)). The endmembers of the hydrogarnet family (grossular, hibschite, and katoite) depend on the degree of substitution (x):
grossular: x = 0
hibschite: 0.2 < x < 1.5
katoite: 1.5 < x < 3.
Hydrogrossular is a garnet variety in which a Si4+ is missing from a tetrahedral site. Charge balance is maintained by bonding a H+ to each of the four oxygens surrounding the vacant site.

So the “hydro” refers to “hydroxide”. Consulting Wikipedia again, hydroxide consists of an oxygen and hydrogen atom held together by a covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. (I will comment on other aspects of the above physical-chemical description in the next Post in this series.) 

If you look up “grossular” in Wikipedia, you discover that “the name grossular is derived from the botanical name for the gooseberry, grossularia, in reference to the green garnet of this composition that is found in Siberia”. 

It wasn’t until someone I met on Gemstone Beach about a year ago showed me some hydrogrossular stones he had just picked up, and actually gave me a couple, that I realised what they looked like and, just as importantly, what they felt like. They feel waxy, not the same kind of smoothness as a quartz stone, which is cool to the touch in contrast to the more “warm” feel of a hydrogrossular.

The Riverton Museum, “Te Hikoi”, has a small room which displays stones from the area, linked to an exercise of stone collecting set up for children on holiday. With the permission of Museum staff, I took some photos of the display and of the drawers of rock samples in this room.

The Southland Museum in Invercargill, now closed due to problems with Earthquake strengthening, had a Minerals display which included a non-smoothed rock of hydrogrossular garnet. 

 On page 34 of the excellent “A Photographic Guide to Rocks and Minerals of New Zealand” by Nick Mortimer, Hamish Campbell and Margaret Low (2011) is an entry on “Hydrogrossular”: 

Five key points made in this entry are:

1) Its chemical composition is hydrous calcium aluminium silicate.  

2) Hydrogrossular garnet hardly ever occurs as good crystals but rather as dense masses.

3) It can be found in the Nelson area as well as around Orepuki.

4) It is one of 13 minerals first described in New Zealand. 

5) It was first identified by Colin Hutton in 1943.

In the next Post in this series, Why are Hydrogrossular Stones Called Garnets?, I will look at the first two points, with Points 3, 4 and 5 the subject of later Posts.

Raining on the Rock

Uluru is probably the biggest rock in this part of the world, a gigantic sandstone boulder on the other side of the Tasman Sea, in the heart of the Australian Outback. It stands 350 metres high, with most of its bulk lying underground, and has a total circumference of nearly 10 kilometres. The remarkable feature of Uluru is its homogeneity and lack of jointing and parting at bedding surfaces. This means that scree slopes and soil have not developed there. These characteristics have led to its survival, while the surrounding rocks were eroded. I have always enjoyed John Williamson’s “Raining on the Rock”, even using it at times in my lectures. The first YouTube clip below is rain falling on Uluru, the second clip is Williamson’s song, the third clip is the first drone footage of Uluru, and the fourth clip is Midnight Oil’s “Dead Heart”. Both pieces of music feature impressive guitar playing. Lyrics are at the end of this post. 






“Raining on the Rock”
by John Williamson

Pastel red to burgundy and spinifex to gold,
We’ve just come out of the Mulga where the plains forever roll.
And Albert Namatjira has painted all the scenes,
And a shower has changed the lustre of his land.

And it’s raining on the Rock,
In a beautiful country,
And I’m proud to travel this big land
Like an Aborigine.
And it’s raining on the Rock.
What an almighty sight to see!
And I’m wishing on a postcard that you were here with me.

Everlasting daisies and the beautiful desert rose –
Where does their beauty come from heaven knows.
I could ask the wedge-tail but he’s away too high,
I wonder if he understands it’s wonderful to fly.

And it’s raining on the Rock,
In a beautiful country,
And I’m proud to travel this big land
Like an Aborigine.
And it’s raining on the Rock.
What an almighty sight to see!
And I’m wishing on a postcard that you were here with me.

It cannot be described with a picture,
The mesmerising colours of the Olgas,
Or the grandeur of the Rock – 
Uluru has power!

And it’s raining on the Rock,
In a beautiful country,
And I’m proud to travel this big land
Like an Aborigine.
And it’s raining on the Rock.
What an almighty sight to see!
And I’m wishing on a postcard that you were here with me.


“The Dead Heart”
by Midnight Oil

We don’t serve your country,
Don’t serve your king,
Know your custom, don’t speak your tongue.
White man came took everyone.

We don’t serve your country,
Don’t serve your king.
White man listen to the songs we sing.
White man came took everything.

We carry in our hearts the true country
And that cannot be stolen.
We follow in the steps of our ancestry
And that cannot be broken.

We don’t serve your country,
Don’t serve your king,
Know your custom, don’t speak your tongue.
White man came took everyone.

We don’t need protection,
Don’t need your hand.
Keep your promise on where we stand.
We will listen, we’ll understand.

We carry in our hearts the true country
And that cannot be stolen.
We follow in the steps of our ancestry
And that cannot be broken…

Mining companies, Pastoral companies,
Uranium companies, Collected companies,
Got more right than people,
Got more say than people.

Forty thousand years can make a difference to the state of things.

The dead heart lives here.

A Batch of Polished Stones from Riverton’s Back Beach

These 78 stones were collected from the Back Beach in March 2019. They were initially tumbled in 320 grit as they were smooth beach pebbles. They were then tumbled in pre-polish and pro-polish tin oxide and burnished in borax. Altogether, these stones were processed  in a 4lb rubber barrel for four weeks. 

Each stone has its own interest and attraction. The following are some of the most intriguing (Note: Some of the close-ups have been manipulated a bit to explore the patterns and colours):



The Back Beach at Riverton faces Stewart Island, some 40 kilometres away across Foveaux Strait, a rough and often treacherous stretch of water. The powerful waves are ideal for smoothing stones as they are tossed back and forth and up and down the beach. 

Visiting the Beach at McCracken’s Rest near Orepuki

I spent a few days based in Riverton early in May, on a stone collection trip. Two of my aims were to spend more time on Gemstone Beach and to explore the beach further to the west, near a place called McCracken’s Rest. I took an extra suitcase down with me so that I could carry more stones home with me than I usually do on the plane. I ended up bringing back 26.5 kgs of carefully selected beach pebbles.

Day One at Riverton saw me drive out to McCracken’s Rest, 36 kilometres from Riverton. This is a roadside lay-by and viewpoint eight kilometres west of Orepuki and Gemstone Beach. 

On YouTube is this clip which gives a good sense of the roadside lay-by (although at 1:24 Stewart Island is misidentified – it is in fact well hidden in the mist – the piece of land referred to as Stewart Island is really the headland between Monkey Island and Cosy Nook, the headland just south of Orepuki – see the third last photo, bottom left, in the group below).  

The beach between Orepuki and McCracken’s lies below cliffs all the way along so access is very difficult. At the viewpoint at McCracken’s Rest, I hopped over the fence and carefully made my way down the steep slope to the beach below. 

The beach at McCracken’s Rest is similar to the beach further south-east, back towards Gemstone Bach and the Waimeamea Lagoon. There is a low bank of stones above the high tide mark, along with a wide scattering of drift wood. Closer to the waves, there are sandy patches and drifts of smaller stones.

I spent two and a half hours there – the day was largely fine and with little wind, which allowed the sandflies to be active. I slowly walked (and fossicked) just over a kilometre north-westwards to the start of the Te Waewae Lagoon (created by the Waiau River trying to find a path to the sea). The actual mouth of the Waiau River can vary in position along this gravel bar, depending on the countervailing forces of the river’s flow and the stones thrown up by the sea.

There seemed to be more slightly larger and less rounded stones here than at Gemstone Beach, and I did not see as many colourful ones. I also found no hydrogrossular garnets although there were fossil worm cast stones.

I collected quite a few stones on the beach but later discarded many of them after careful re-examination. This was partly because I found much better stones later at Gemstone Beach and on the Riverton beaches. I still ended up bringing home 2.3 kilograms of stones from the beach between McCracken’s Rest and Te Waewae Lagoon.

Before returning to Riverton, I drove up to Fishing Camp Road, about two and a half kilometres north-west of McCracken’s Rest, and drove along it to the shores of the Te Waewae Lagoon. This brought me to the landward side of the lagoon, near a handful of fisher huts and a boat ramp. The stones there were dirty and slimy and uninteresting – but one could gaze across the lagoon at the gravel bar separating the lagoon from the sea and see the kind of interesting ones to be found between there and Gemstone Beach.

Nine Milestones at Journey’s End

For a number of years before I retired from the University of Waikato, I assisted with the supervision of Gemma Piercy-Cameron’s PhD thesis. Gemma was finally successful in completing her grand project, Baristas: The Artisan Precariat, a few months ago. Currently, Gemma is a Lecturer in Sociology and Social Policy at the University of Waikato (see her Staff Profile). I presented her with nine milestones to mark her accomplishment.

The following letter accompanied Gemma’s milestones (photos added here):

Why Nine Stones?
Nine is seen to have philosophical significance, due to its unique numerical attributes. In the Hebrew tradition, for instance, Nine represents truth, since it reproduces itself when multiplied. Multiply any number by 9, then add the resulting digits and reduce them to a single digit, it always becomes a 9 again, e.g., 6 x 9 = 54, 5 + 4 = 9; 23 x 9 = 207, 2 + 0 + 7 = 9. Another attribute of Nine is that when added to any other number and then that number is reduced to a single digit, it always comes back to itself, as if nothing was added at all. For example, 5 + 9 = 14, 1 + 4 = 5; 7 + 9 = 16, 1 + 6 = 7. Nine is the Triple Triad, consisting of three times three, and so is seen as symbolic of completion, fulfillment, attainment, the beginning and the end, the ultimate whole number. Appropriate to recognise the completion of a PhD!

Stone #1 “Coffee”

Unknown type, collected at Riverton (Southland) July 2017, polishing completed September 2017. Polishing brought out the creamy swirl that reminded me so strongly of coffee and latte art that I knew it was destined for you.


Stone #2 “Positioned Sparkle”

Mica-rich pegmatite rock, collected at Joyce Bay (near Charleston, Buller District) March 2017, unpolished. Your thesis reflects who you are, and sparkles as it is turned to be viewed from different positions. Different things will be seen in it depending on who views it from which position.


Stone #3 “Effort”

Metamorphic mudstone, collected at Riverton July 2017, polishing completed September 2017. This stone started millions of years ago as a number of sediment layers, being compressed by weight and heat. Your thesis consists of layers of effort and activity, building on each other, one layer being the foundation for the other. Over time, effort becomes more focused, refined, productive, until completion is reached.

Henderson Bay, Riverton


Stone #4 “Complexity”

Jasper, with silica, collected at Riverton February 2018, polishing completed August 2018. Reality is complex and resists analysis. Analysis is hard labour.


Stone #5 “Depth”

Pale green Quartzite, collected at Orepuki (Southland), April 2016, polishing completed November 2017. Depth of understanding and insight is gained by multi-method qualitative research.


Stone #6 “Found Worthy”

Banded Agate, collected on Birdlings Flat (Canterbury) June 2016, polishing completed September 2016. Agate is formed from quartz crystals growing in layers so small they can barely be seen. The layers build up to fill cavities in sediments left by gas bubbles in volcanic rocks. This particular banded agate is very unusual (the only one of its kind I have found) – when held up to the light, it is apparent that the bands are not smooth but have intricate and delicate lace-like waves in them. This stone will have originated in the Alps, been washed down a Canterbury river, and swept along the coast to be deposited on Birdlings Flat which abuts Banks Peninsula. Your thesis has survived close examination in the light of others’ assessments, and has been found to be worthy of scholarly esteem.


Stone #7 “Patterns”

Unknown type (possibly a type of schist?), collected at Riverton February 2018, polishing completed August 2018. Research identifies patterns and layers and makes sense of them for others.


Stone #8 “It takes time to construct an interesting story”

Argillite metamorphic mudstone, with fossil worm casts, collected at Orepuki February 2018, polishing completed August 2018. This argillite started as mud under the sea 250-280 million years ago. The interesting linear features were left behind by ancient worms who had ingested lighter coloured mud. All pieces of scholarly writing, including your thesis, are like fossils of your thoughts at a particular period of time, persisting in existence even as you go on to other thoughts and activities.


Stone #9 “Well Travelled and Wide Ranging”

Quartzite, stained with iron, collected at Budleigh Salterton (Devon, England) May 2018, polishing completed August 2018. These Devon stones are identical to rocks found in Britany in France. 200-250 million years ago, Britany was mountainous and rivers drained from it northwards across the Triassic desert. The quartzite rocks were tumbled into pebbles and eventually deposited as pebble beds outcropping on cliffs at the beach of Budleigh Salterton village. Good PhD research takes time, has gone places, and has a broad base of experience and reflective thought.

Some Recently Polished Stones from Riverton

Just out of a 3lb borax burnishing tumble, 43 newly-polished stones from a Riverton beach.

I collected these stones at a beach I call the “Beach Past the Back Beach” at Howells Point, Riverton, in March 2019. When you reach the end of the road at Howells Point, there is a track up the sand dunes. Down the other side is this beach, which stretches for maybe 900 metres further westwards. Many of these stones were found at the far eastern corner, the area circled in this photo:

Riverton’s “Back Beach” is the stretch along the last part of the road that winds along the coast. If you walk over the dunes from there, you come to the “Beach Past the Back Beach”.

These are not particularly spectacular stones, though some are really interesting. Not all have polished perfectly, some have scratches and holes in them, but I polished them because of their intriguing colours and patterns.

Photos of a selection of these newly-polished stones: