Diamond (once known as adamant) is the hardest naturally occurring mineral, the purest form in which the element carbon is found, and the most important of the gem stones. Carbon crystallizes to form diamonds in forms belonging to the regular or cubic system, the most common being the regular octahedron and rhombic dodecahedron (twelve faces). The finest diamonds are colourless, perfectly clear, and pellucid. Such are said to be of the finest water. But diamonds are often blue pink, green, or yellow, and such are highly prized if of a decided and equal tint throughout. The hardness of the diamond is such that nothing will scratch it, nor can it be cut but by itself. The value of a diamond is much enhanced by cutting facets upon it inclined at certain angles to each other so as to produce the greatest possible play of colour and lustre. What is called the brilliant cut best brings out the beauty of the stone. Its upper or principal face is octagonal, surrounded by many facets. But this form of cutting requires an originally well-shaped stone. For other diamonds the rose cut is used. In this form six triangles are cut on the top so that their apices meet in a point called the summit. Round this are disposed other facets. Stones which are too thin to cut as rose-diamonds are cut as table-diamonds, which have a very slight play of colour. Diamonds have a relative hardness of 10. In jewellery terms, diamonds are measured in carats, with one carat weighing 0.2 grams, so a five carat diamond weighs one gram.
The art of cutting and polishing the diamond was unknown in Europe until the 15th century, and the stone itself was not nearly so highly valued in the middle ages as the ruby. Diamonds are valuable for many purposes. Their powder is the best for the lapidary, and they are used for jewelling watches, as lenses for microscopes, and in the cutting of window and plate glass. When used as a glazier's tool the diamond must be uncut. Inferior kinds of diamonds are also extensively used by engineers in rock-boring and by copperplate engravers as etching-points. Diamonds are obtained from deposits of various kinds, mostly alluvial (sands, clays, etc), being separated by washing. They have been found in India, Borneo, and other parts of the East; sometimes in North America and Australia; Brazil has produced large numbers; but the chief diamond-field since 1900 has been in South Africa, the centre being Kimberley. Diamonds were discovered here in 1867, and since then the output has been immense. The diamonds were originally obtained by mere surface workings, but from the late 19th century the excavations have been carried down to considerable depths. 'River diggings' are also carried on the banks of some of the rivers. Some of the South African diamonds are very large. One of them, the Cullinan diamond, discovered in 1905, is a monster of 3025 carats, of very good colour, being by far the largest diamond known. A celebrated diamond is the Koh-i-noor (Mountain of Light), an Indian stone belonging to the British crown. Its history extends over five or six centuries. It weighed at one time 280 carats, but by cutting has been reduced to about 106. The Orloff diamond, formerly belonging to the Emperor of Russia, weighs 193 carats; the Pitt diamond, among the French crown jewels, 136.5 carats. Research Diamond
Fluorite (calciumfluoride) or fluorspar or Derbyshire-Spar is a mineral occurring in many different colours, found in veins where it's the main mineral or with metallic ores, especially lead and silver. It generally occurs massive, but crystallizes in simple forms of the monometric system: the cube, octahedron, dodecahedron, etc, and in combinations of the cube and octahedron. It is common in dolomites and limestone and is associated with many different minerals. From the general prevalence of a blue tint in the Derbyshire specimens it is there known as Blue-John. It is often beautifully banded, especially when in nodules, which are much prized for the manufacture of vases, and it is made into a great variety of articles, chiefly ornamental. It is used chiefly as a flux in the making of steel, and also for enamelling, and in the preparation of hydrofluoric acid. It is a fluoride of calcium with the formulae CaF2 and a relative hardness of 4. Research Fluorite
Garnet is a widely distributed group with several minerals. They are found in both metamorphic and igneous rocks. The chief use of red transparent garnets are as an inexpensive gem stone, however, much is used as an abrasive material. They have the formulae A3B2(SiO4)3 and a relative hardness of 8. The commonest colour of garnet is red and the lustre is vitreous. The dodecahedron and trapezohedron are the common forms. There are also white, green, yellow, brown, and black varieties. The garnet is a silicate, the bases being aluminalime (grossularite, essonite, or cinnamon stone), or alumina magnesia (pyrope), or alumina iron (almandine), or aluminamanganese (spessarite), or iron lime (common garnet, melanite, allochroite), or chromiumlime (ouvarovite, the colour emerald green). The garnet was, in part, the carbuncle of the ancients. Garnet is a very common mineral in gneiss and micaslate. Research Garnet
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Matt and Leela Probert
Abbreviations
Research Dodecahedron
