In my first years of gem hunting in Yarrow Creek I occasionally came up with pieces of a mineral which looked like black spinel but differed in that they showed a green to orange-brown colour when held up to the sun. The best specimens were transparent and gemmy.
As I had done before with specimens of pink sapphire, I sent a few samples to Hylda Bracewell, the highly regarded gemmologist then living in Torrington NSW. Hylda concluded that they were enstatite with some examples approaching hypersthene in their properties.
These minerals are members of the pyroxene family, which are commonly found in basic to ultrabasic igneous rocks and some metamorphic rocks. Their chemical formula can be summarised as (AB)₂Si₂0₆, where A and B are metals. In the family of which enstatite and hypersthene are part, the metals are magnesium (Mg) and iron (Fe). Pure enstatite contains no iron, only magnesium, hypersthene contains both iron and magnesium.
An important question to be answered is this: where does this mineral come from? Not from the local granites, which appear to contain no pyroxene minerals. Not from the same volcanic source as corundum, zircon and spinel, since it is not found with them in most local creeks. Perhaps from the gabbro dyke which outcrops just upstream from Garnet Corner, some other dyke yet to be identified, or from xenoliths in the granite.
These minerals are not especially hard or dense, but hard and dense enough to end up in the sieve centre along with the sapphire, zircon, garnet and spinel which make up the bulk of the concentrate. Enstatite has a hardness of 5.5 on Mohs’ Scale and a specific gravity of 3.2 – 3.3. They can be cut as gems but need to be protected from undue wear. The colours are not particularly attractive and difficult to see in a set stone.
Despite its shortcoming as a gemstone, a piece of rich green enstatite is a good find and many amateur cutters would like to work on a Yarrow Creek stone if they could find one.
|Facetted enstatite 1.5 carats|