Just under a year ago, my niece Diane bought a new house in Christchurch. While doing work in the garden, she found a stone that she thought was potentially interesting. She showed it to me when I was visiting her and I recognised it as jasper. The stone was in a rough condition. It was far from smooth, and running through the middle was a brittle and pitted silica band. The photos below are of the stone after it was washed and allowed to dry:
I thought that tumbling it would at least bring out the colour, so I offered to take it away to work on it.
What is jasper? Jasper is a well-known red stone found on many beaches in New Zealand, especially in the South Island. I have found it on beaches like Birdlings Flat in Canterbury, and at Riverton and Gemstone Beach in Southland. It tends to stand out amongst other stones because of its deep red colour:
I once found a rock in a track on the farm I grew up on, The Mains near Waikaka. When trying to identify it, I thought first of all that it was chert but it turned out to be jasper (Minerals.net states that “when Jasper is dull and lacking interesting colors or patterns, it is not Jasper but rather Chert”). I eventually broke it up, with difficulty – to put it technically, jasper “fractures conchoidally” (like flint – Geology.com points out that jasper, chert and flint are very similar, all being varieties of opaque microcrystalline quartz). When polished, the pieces of the rock I found had a very glossy, almost waxy, quality.
I later discovered that jasper is very common around the Waikaka area, being part of the gold-bearing quartz gravels of the area – see the Tumblestone Posts “Jasper Stones and Petrified Wood, Shepherd’s Creek, Waikaka” and “Waikaka’s Auriferous (Gold-Containing) Quartz Gravels“.
In Riverton, outside the museum Te Hikoi, a number of rocks from Southland are on display. One of them is a large piece of rounded jasper. Inside the museum, a number of smaller jasper stones are displayed, including a dark green one.
When I recently visited Jack Geerlings in Winton, who has an extensive collection of stones and rocks from southern New Zealand, I noticed he had a number of pieces of jasper (see photos below). He had collected some from the Coromandel Peninsula, in the North Island, and generously gave me a piece. Outside his shed, where he had a number of rocks and boulders on display, a rock of red and green jasper can be seen. And inside his shed, he showed me a polished stone of orbicular jasper. In her book “Collecting Rocks, Gems and Minerals” (3rd edition, 2016), Patti Polk has an extensive section on different types of jasper. On pages 121 and 122, in her entry on orbicular jasper, she states: “Orbicular jasper is generally a type of highly silicified rhyolite or tuff that has quartz and feldspar crystallized into radical aggregates of needle-like crystals that form orbicular ir spherical structures.” The photos below include page 122 from her book, illustrating some of the variants of orbicular jasper.
To find out more about jasper, I consulted one of my books, “The Illustrated Guide to Rocks and Minerals” (2015) by John Farndon. On page 204, he notes that jasper is a red, green or yellow variety of chalcedony. Chalcedony arises when quartz crystals forms at low temperatures in volcanic cavities. The crystals can be so small that they are visible only when magnified, which is the meaning of the term “cryptocrystalline”. The general name for cryptocrystalline quartz (also called “microcrystalline”) is chalcedony, which comes in a vast array of colours and patterns. As Farndon notes, it includes blood-red carnelian, wine-red jasper, brown-banded agate, green-moss agate, apple-green chrysoprase, and black and white onyx. The colour depends on what minerals seep into the rock. The presence of iron causes the red colour in jasper. In fact, jasper is often referred to as a coloured form of opaque agate, agate being one of the major types of chalcedony (see, for example, the wikipedia entry on chalcedony).
Mindat.org has a lot of photos of different variations of jasper. Below are also photos of some different types of jasper, with the one on the right showing rough and tumbled stones.
To get back to Diane’s jasper stone: After tumbling for just over a week (9 days and 19 hours, to be precise) in 220 silicon carbide grit (along with other stones), followed by five and a half hours tumbling in water and sunlight soap, the stone’s colours emerged much more clearly. Nevertheless, it was still rough in places, with the middle band of silica especially pitted:
However, the stone showed promise. I next tumbled it in 320 silicon carbide grit for another week (seven days and one hour to be exact), followed by 21 hours in sunlight soap. There was a small improvement in the surface condition of the stone, though of course the pits in the middle silica band were too deep to erode away:
I next tumbled the stone in pre-polish tin oxide for five days, followed by a 21 hour soap tumble. Then it spent nearly 12 days in pro-polish tin oxide, finally spending just over four days in a borax burnishing tumble. Unfortunately, these stages seemed to bring out various lines of weakness in the stone, and it failed to take a polish. I didn’t think that further tumbling would improve it:
While I found this disappointing, it is not unusual for some jasper. Many of the jasper stones found on the beach have a chip out of them or some kind of crack. There is often a brittleness in jasper that gives rise to this.
At least Diane now has a much cleaner and more colourful stone.