Tigers eye variety of quartz, SiO2 is formed from by the parallel intergrowth of quartz and altered amphibole fibres which have mostly changed to limonite. Limonite is an iron mineral which gives Tigers eye it's golden brown colour. This specimen is in it's natural form, but would be much more striking in appearance had it been polished, making its chatoyant properties more obvious. The only information that I have on its origin is New Zealand.
Muscovite is the most common form of mica. It is known as insinglass, potash mica, or common mica, and is a complicated hydrated phyllosilicate of aluminium and potassium. The basic formula is KAl2(AlSi3O10)(F,OH)2 The very thinly laminated structure of mica, is very obvious in this specimen, which has been collected from Two waters, Liskeard, East Cornwall.
Gypsum, Calcium sulphate dihydrate, CaSO4 . 2H2O As described in other gypsum references, gypsum is very variable in colour and appearance. These two specimens are from the same location, Whyalla, Eyre peninsula, in South Australia. One specimen is a dark brown compact mass with two smooth faces, and the other specimen is much lighter in colour, and an untidy mass of elongated prismatic crystals.
Sodalite is named because of it's soda content. It is a tectosilicate mineral with the formula Na8 ( Al6 Si6 O24 ) Cl2 , and is classed as a feldspathoid mineral. It has the appearance of a rock, but it is a mineral, and when polished can be used for making jewellery. Unfortunately, it's origin can only be loosely attributed to New Zealand.
Gypsum, Calcium sulphate dihydrate, CaSO4 . 2H2O is an evaporite mineral. The variety of this specimen is known as selenite, and it probably derives it's deep green colour because of the locality of its origin, which is the Lubin copper mine in Poland. Gypsum is very variable in colour and habit, and crystals can vary in size from a few millimetres to several metres, dependent on depositional conditions and proximity of other minerals. The conditions at Lubin have produced this instantly recognisable selenite variety of small green acicular crystals.