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.
Biotite is named after the French naturalist J.B.Biot. Crystals are tabular or occur as lamellar aggregates or disseminated flakes.
Calcite is a trigonal mineral crystallising in a variety of forms. Very common, it can be found in many different environments - it is the major component of several calcareous sedimentary rocks, such as limestone, chalk and the metamorphic rock marble.
Chlorite is not a member of the mica group but has the same perfect basal cleavage. The plates are easily distinguished from mica by their green colour and lack of elasticity.
Fossils are mostly found in sedimentary rocks such as, shale, sandstone and limestone. They are rarely found and difficult to recognise in metamorphic and volcanic rocks and do not occur in igneous rocks. Trilobite, which are an extinct group, hold special fascination. A Trilobite skeleton consists of a head plate (cephalon) which has 2 separate cheek plates thinning to a point tailwards, a tail plate (pygidium) and below the head, a mouth cover plate (hypostoma). Normally the fossillsed plates become separated as they have done in the Territory's Maloney Creek area. Fossil finds in this area indicate that Trilobites existed in the Territory some 485 million years ago.
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.
Hornblende is monocline and crystals are usually prismatic, often with 6-sided cross sections; but it also occurs in massive granular or fibrous forms. A very common rock-forming mineral it is found in both igneous and metamorphic rocks and belongs to the amphibole group.
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.
The Mica belongs 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.
Muscovite derives its name from the old name for Russia, muscovy. It usually occurs as lamellar masses or small flakes. Muscovite is colourless or pale green, grey or brown, transparent of translucent with vitreous lustre. It is the common mica widely used as electrical and heat insulating material.
The primary sources of many gems are in the pegmatite vein systems often associated with the late - stage igneous activity of granite emplacement. Natural jewel boxes, pegmatites provide the space and the chemical isolation that are necessary for the formation of pure mineral crystals. They yield many gems, including topaz, beryl and tourmaline.
A member of the phylliosilicate family, prehnite is found in low grade metamorphic lavas, fissures, joints and other cavities of rocks. Named after the Dutch Colonel, H. von Prehn, prehnite is sometimes used as a precious stone.
Quartz (the most common mineral in the earth's crust)
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.
Sphene is a calcium titanosilicate. The name originates from the Greek word sphen, "a wedge", alluding to the typical shape of crystals of the mineral. Transparent crystals are sometimes cut as gems - stones are fiery and brilliant, Sphene is widely distributed as accessory mineral in intermediate and acid igneous rocks and also associated pegmatites.
If hermatite flakes are present in quartz in more or less parallel positions, the variety of aventurine quartz is produced. The same effect occurs in sunstone or aventurine feldspar, the shimmering optical effect being produced by orientated inclusion of hematite, ilmenite or limonite.
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.
Zebrastone is a reddish brown, iron-bearing siltstone with a fine texture and a base of white to grey colour. Within the white rock, the dark red, shaped sphere, rods and sheets of siltstone give rise to the distinctive patterns best seen on a sawn face. The colour changes in the rock are though to be due to differential leaching. The material is cohesive, unfractured and may be carved easily.
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.