Quartz, Amethyst var.

Quartz, Amethyst var.

Quartz, silicon dioxide, SiO2 has many varieties, dependent on conditions in which it is formed, or the inclusions of trace minerals. Amethyst quartz gains it's purple colour from irradiation, or trace amounts of iron, or both. This specimen from south Australia has a distinctive boundary between two episodes of growth.  The first episode has deposited quartz with just a hint of purple, and the second episode a growth of which is deep purple. This cut and polished slab reveals the outline of the tops of crystals from the first deposition. The inset picture shows the reverse of the specimen.

Jasper, Tiger iron var.

Jasper, Tiger iron var.

Tiger iron jasper is a gemstone variety of jasper. It is a member of the quartz group, SiO2 and is a parallel growth of quartz, and amphibole fibres. The combination causes chatoyency, which is a term used to describe the reflective property that makes it so popular for use in making jewellery. One specimen is in its natural appearance, and the other specimen has been cut and polished to reveal a hidden beauty. Both specimens were collected from the same site in western Australia by Jim Nicholls, founder member of the club.  

Calcite, Creigiau quarry.

Calcite, Creigiau quarry.

Calcite, calcium carbonate, CaCO3.  Two more specimens of calcite from Creigiau quarry, South Wales. These and all other specimens in my collection were collected at one visit. The main picture is a cluster of several untidy twinned crystals. and in small pockets between them are small groups of small single bladed crystals with the appearance of flattened " rhombs ". The inset picture shows a single crystal which is not on the same scale, but which has been reduced in size to fit this illustration.    

Fulgarite, Silica var.

Fulgarite, Silica var.

This specimen is a fulgarite,  which has been created by the action of lightning. When lightning strikes the ground it vaporizes the sediment, soil, or sand in the immediate area, creating tubes with a smooth interior.  Fulgarites can be several meters long, and sometimes are branched. This particular fulgarite from the U.S.A. is the partial tube of a sand fulgarite.  Sand fulgarites  are likely to be a silica rich mineraloid, lechatelierite  

Jasper, Mookaite var.

Jasper, Mookaite var.

Mookaite is a local name attributed to this jasper, SiO2.  It is so named because it is from Mooka creek, Carnarvon Shire, near the Kennedy ranges in western  Australia. It's geological origin is from the Windalia Radiolarite, a lower Cretaceous siltstone which outcrops over the Carnarvon basin. It was collected, cut and polished by founder member of the club, Jim Nicholls, while working in Australia.