In addition to the well known lilac and purple amethyst cactus quartz, the region of Boekenhouthoek in South Africa has also started producing cactus quartz that range in shades of yellow from lemon yellow to gold yellow. The remarkable yellowing of the crystals is due to a coating of Goethite.
Since goethite is an oxide of iron, you’d think that the coloring would be easily removed with oxalic acid ,but not so. I experimentally put a couple of crystals in an acid bath for 2 weeks and the color hardly changed!
|Dimensions||4 × 3 × 4 cm|