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Gems and Minerals Dictionary

Amethyst is the violet to purple variety of macrocrystalline quartz that is highly regarded as an ornamental stone. It is characteristic of amethyst that quite often only the tips of the crystals are deep coloured, the remainder grading into milky quartz or rock crystal. Hence, although amethyst us always violet in hue, the range of colour is wide and may vary from almost colourless to glorious purple.

Apatite derives its name from the Greek word apate, "deceit", referring to its misleading similarity to other minerals. Apatite crystals are sometimes cut as gems. Apatite is an accessory mineral found in a wide range of igneous rocks, including pegmatites and high-temperature hydrothermal veins.

Beryl is beryllium aluminium silicate, named for its content of the rare metal beryllium, although it is particularly noted for its properties as a gemstone. Beryl occurs chiefly as an accessory mineral in both granite and granite pegmatites; in the latter rocks crystals may grow to a considerable size. There are 4 gem varieties of which aquamarine is the most common found in the Territory. Its blue-green colour is caused by traces of iron.

Garnet is a typical mineral of metamorphic rock, usually forming isolated crystals disseminated through the rock. Industrially, garnet is used as an abrasive. Garnet is really the name for a group of minerals, with a common crystal habit and chemical composition. There are 6 members of the garnet group of which Almandine, an iron silicate, is most common to the Territory. It is named after Alabanda, a town in Turkey, where mining, cutting and polishing of the stones was practiced in ancient times. The colour is usually dark red inclining to violet-red.

Gold is steeped in history and has long been viewed as the most highly prized of metals. Its malleability, relatively low melting point and chemical inertness have since time began made it ideal for works of art like the mask that covers the upper body of the mummified 14th Century Egyptian King Tutankhamen. Gold is cubic and has been found in well-formed octahedral and dodecahedral crystals, although it usually occurs as dendritic growth and occasionally as rounded masses (nuggets). Gold occurs in many types of rocks but is common in small amounts in hydrothermal veins associated with quartz. Gold-bearing quartz id often of the milky variety.

Magnetite is an iron oxide, which is so named for its strong magnetic property. It is a cubic mineral with the spinel type of structure. It can be distinguished from hematite by its streak - magnetite being black and hematite red. It is an important ore of iron, being found as a high-temperature accessory mineral in igneous and metamorphic rocks and in�� sulphide veins.

Mica - The micas belong to a group of minerals whose most obvious characteristic is the perfect cleavage, which means the minerals may be split into leaves thinner than sheets of paper. These leaves are flexible and often hexagonal in outline. The micas are monoclinic potassium alumino-silicates with sodium, lithium, magnesium or iron also present in certain varieties.

Microcline received its name from the Greek words micro, "small", and kleinen, "incline", referring to the small inclination of the third crystallographic axis which makes microcline a triclinic mineral. Amazon stone is a green variety of microcline, and moonstone has a bluish opal essence.

Quartz is colourless when pure but the presence of impurities gives a whole range of colours: purple - amethyst, smoky - cairngam, yellow - citrine, white - milky quartz, black - morion, pink - rose quartz, colourless - rock crystal. The type of chrystalline quartz may occur as well formed and sometimes enormous prismatic 6 - sided crystals with rhombohedral terminations or as compact and concretionary masses: whilst cryptocrystalline quartz occurs as compact microcrystalline masses, or as mamillary aggregates of nodules often with more or less concentric bands of colour.

The 2 varieties typical to the Territory are: Agate - variegated chalcedony with coloured bands in concentric form usually following the outline of the cavity in which the mineral has formed. Colours range from shades of white, grey, green, brown, red or black. Moss agate of mochastone is chalcedony containing dark moss-like or dendritic forms. Ribbonstone is a variety of agate. Jasper - chemical sedimentary rock, possibly precipitated from volcanic solutions in the deep ocean away from the influence of any coastline. It is typically opaque and is coloured by large amounts of pigments usually iron oxides. Jasper is usually bright red or brown, but may also be grey, green, yellow or black. Colours may be intermixed to give splotched, banded, spotted, brecciated or orbicular effects.

Tourmaline is one of the most attractive of the silicate minerals - transparent crystals have been cut as gems for many centuries. It represents a group of complex silcates which form typically elongated and vertically striated prismatic crystals or aggregates of parallel or radiating crystals. Gem varieties occur in yellow, green, blue or watermelon (green and red) colours.

Zircon is a silicate occurring as stubby prismatic tetragonal crystals. It has a strong double refraction and a high dispersion or 'fire', thus making for very attractive cut stones. Zircons occur in a range of attractive shades from colourless to brown, orange, red or yellow.