Alumina (Al2O3) is the single oxide of the metal aluminium. As found native it is called corundum, when crystallized ruby or sapphire, when amorphousemery. It is next to the diamond in hardness. In combination with silica it is one of the most widely distributed of substances, as it enters in large quantity into the composition of granite, traps, slates, schists, clays, loams, and other rocks. The porcelain clays and kaolins contain about half their weight of this earth, to which they owe their most valuable properties. It forms compounds with certain colouring matters, which causes it to be employed in the preparation of the colours called lakes in dyeing and calico-printing. It combines with the acids and forms numerous salts, the most important of which are the sulphate (Alum) and acetate, the latter of extensive use as a mordant. Research Alumina
Asteria is the name applied to a variety of corundum, which displays an opalescentstar of six rays of light when cut with certain precautions. The name is also applied to the cat's-eye, which consists of quartz, and is found especially in Sri Lanka. Research Asteria
Chromite is a black sub-metallic mineral consisting of oxide of chromium and iron. It is slightly magnetic and the only ore of chromium. A common constituent of peridotite rocks and the serpentines derived from them. Also associated with corundum. One of the first minerals to separate from a cooling magma. Chromium is widely used in metal plating and in stainless steel. It has the formulae FeCr2O4 and a relative hardness of 6. Research Chromite
Corundum is a form of aluminium oxide common as an accessory mineral in metamorphic rocks and as an original constituent of certain igneous rocks. Pure corundum is colourless, but colour differences due to the presence of other elements give rise to several varieties of gem, notably, ruby and sapphire. The deep red ruby (corundum tainted with chromium) is one of the most valuable gems, second only to emerald and diamond. Sapphire is corundum tainted with titanium.
Corundum has the formulae Al2O3 and a relative hardness of 9. Research Corundum
Cyanite or kyanite is an accessory mineral of the garnet family found both massive and in regular crystals in gneiss and micaschist. It often found with garnet and corundum. It is used in the production of refractory porcelains. It has the formulae Al2SiO5 and a relative hardness of 7. Its prevailing colour is blue but of varying shades. Research Cyanite
Danburite is a mineral and gemstone which occurs with feldspar in dolomite, granite pegmatites, and non-marine evaporates. It is commonly associated with apophyllite, cassiterite, calcite, chalcopyrite, corundum, fluorite, pyrite, sphalerite and quartz. Danburite is a borosilicate of calcium and has the formulae CaB2(Si2O4)2 and a relative hardness of 7. It was named after the place where it was discovered - Danbury in Connecticut. Danburite is often colourless, but may sometimes appear pale yellow, pale pink, brown or white and nearly always has a striking glassy or greasy sheen. Research Danburite
Emery is an impure fine-grained form of corundum (aluminiumoxide) of blackish or bluish-grey colour, chiefly found in shapeless masses and mixed with other minerals.in conjunction with iron oxides and spinel. Emery is very hard; is infusible, and is not attacked by acids; has the formulae Al2O3 employed widely as an abrasive: it is employed in cutting and polishing precious stones; in smoothing the surface of the finer kinds of lenses preparatory to their being polished; in the polishing of marble; by cutlers, locksmiths, glaziers, and other artisans. For all these purposes it is pulverized in large iron mortars or in steelmills, and the powder, which is rough and sharp, is carefully washed and sifted into eight or ten different degrees of fineness. Emery-paper and emery-cloth are made by laying a thin coat of glue upon the fabric, and dusting the emery from a sieve of the required size. Typically emery consists of 60 percent corundum and 40 percent iron oxide in the form of magnetite. Research Emery
Gems, or precious stones, are sometimes found crystallized in regular shapes and with a natural polish, more commonly they are of irregular shapes and with a rough coat.
The term gem often denotes more particularly a stone that is cut, polished, or engraved, and it also includes pearls and various artificial productions.
The first and most valuable class of gems includes diamonds, emeralds, rubies, sapphires, and a few others; the second class includes the amethyst, topaz, garnet, etc;
while agate, lapis-lazuli, cornelian, etc, though much used for ornament, can scarcely be called gems.
The fabrication of artificial gems became an important industrial art during the 19th century. The base of one class of imitations is a peculiar kind of glass of considerable hardness, brilliancy and refractive power called paste or strass, which is distinguished from ordinary glass by the presence of 50 per cent of oxide of lead among its constituents. When the strass is obtained very pure it is melted and mixed with substances having a metallic base, generally oxides, which communicate to the mass the most varied colours.
Another class, called semi-stones or doublets, are made by affixing thin slices of real gem to an under part of strass by means of an invisiblecement. In some cases an imitation is made by setting uncoloured strass or quartz in jewelry with some coloured 'foil' at the back of it.
At the end of the 19th century attempts were made with a fair measure of success to manufacture true gems by artificial processes. The French chemists Becquerel, Ebelman, Gaudin, Despretz, and others did much in this direction.
In 1858 Deville and Caron communicated to the Academy of Sciences, Paris, a process for the production of a number of gems of the corundum class, such as rubies, sapphires, etc. The process essentially consisted in exposing the fluoride of aluminium, together with a little charcoal and boracic acid, in a plumbagocrucible protected from the action of the air, to a white heat for about an hour. Many experiments with a view to producing diamonds artificially have also been made. From hydrocarbons, subjected to a very intense heat and enormous pressure, minute crystals, differing from natural diamonds in no respect save brilliancy, have been produced. Artificial diamonds are now widely used in industry.
In art and archaeology the term gem is usually applied to a precious stone cut or engraved in ornamental designs, or with inscriptions. Stones on which the design is raised above the general surface are called cameos; those having the design sunk below the surface are called intaglios. Early specimens of cut gems are seen in the scarabaei or beetle-shaped signets worn in rings by the ancient Egyptians. Among the Greeks, Etruscans, and Romans gem-sculpture held a high place, reaching its highest point under Augustus. Modern gem-engraving dates from the beginning of the 15th century, the chief seats of the art being Italy and Germany. Rome becoming the headquarters of the seal-engraving art. The traditional tools of the engraver consist of a lathe, and a series of little rods with heads of different shapes, all of which can be adjusted to the lathe. The axis of the lathe is pierced at the centre with an orifice, into which the tools for cutting the stone are firmly fixed by means of a screw. The engraver wets the extremity of the mounted rod with diamond dust made into a paste with oil (traditionally olive oil), and as the wheel is in motion he applies the stone, firmly cemented to a piece of reed, to the revolving tool. The diamond dust enables the tool to cut into the stone with ease. As the design is frequently very elaborate and of the greatest delicacy, the tools are necessarily multiform. The stones used for cameo-cutting often exhibit layers of different colours, so that the raised design has a tint distinct from the ground. Intaglios are very often executed in transparent stones, and the subjects treated in this manner are more limited in number. They are chiefly such as seals, devices, coats of arms, etc. Research Gems
Hardness refers to the resistance of a smooth surface of a rock or mineral to scratching. It is determined by the binding force of atoms within the crystal structure and is measured according to Moh's ten division scale of hardness, ranging from 1 the softest, to 10 the hardest: 1) talc 2) gypsum 3) calcite 4) fluorite 5) apatite 6) orthoclase 7) quartz 8) topaz 9) corundum 10) diamond.
Materials, according to this arrangement, which are scratched by quartz and are not scratched by felspar are said to have a hardness between 6 and 7.
The Probert Encyclopaedia was designed, edited and programed by
Matt and Leela Probert
Abbreviations
Research Adamantine Spar
