Gemstone colour unless skillfully estimated and wisely used in conjunction with other properties is an unreliable guide in identifying a gemstone. However, it is a great help and serves sometimes to narrow down the chase, at the start, to a very few species. In order to make use of colour when identifying a gemstone you must have actual acquaintance with the various gem materials and their usual colours and shades and an eye trained to note and to remember minute differences of tint and shade. The suggestions which follow as to usual colours of mineral species must only be used with discretion and after a deal of study of many specimens of each of the species.
Starting with the first colour of the visible spectrum, red, and omitting a number of rare minerals, we have corundum ruby, garnet of various types, zircon, spinel, and tourmaline. These five minerals are about the only common species which give us an out-and-out red stone. Let's now consider the distinctions between the reds of these different species. The red of the ruby, whether dark (Siam type), blood red (Burmah type), or pale (Ceylon), is more pleasing usually than the red of any of the other species. Viewed from the back of the stone (by transmitted light) it is still pleasing. It may be purplish, but is seldom orange red. Also, owing to the dichroism of the ruby the red is variable according to the changing position of the stone. It therefore has a certain life and variety not seen in any of the others except perhaps in red tourmaline, which, however, does not approach ruby in fineness of red colour.
The garnet, on the other hand, when of fire-red hue, is darker than any but the Siam ruby. It is also more inclined to orange red or brownish red - and the latter is especially true when the stone is seen against the light. Its colour then resembles that of a solution of "iron" such as is given as medicine. The so-called "almandine" garnets (those of purplish-red tint) do not equal the true ruby in brightness of colour and when held up to the light show more prismatic colours than the true ruby, owing to the greater dispersion of garnet. The colour also lacks variety (owing to lack of dichroism). While a fine garnet may make a fair-looking "ruby" when by itself, it looks inferior and dark when beside a fine ruby. By artificial light, too, the garnet is dark as compared with the true ruby, and the latter shows its colour at a distance much more strongly than the garnet.
The red zircon is rare. (Many hessonite garnets are sold as red zircons in the trade. These are usually of a brownish red.) The red of the red zircon is never equal to that of the ruby. It is usually more somber, and a bit inclined to a brownish cast. The dispersion of zircon, too, is so large (about 87 percent of that of diamond) that some little "colour-play" is likely to appear along with the intrinsic colour. The luster too is almost adamantine while that of ruby is softer and vitreous. Although strongly doubly refracting, the red zircon shows scarcely any dichroism and thus lacks variety of colour. Hence a trained eye will note these differences at once and not confuse the stone with ruby.
Spinels, when red, are almost always more yellowish or more purplish than fine corundum rubies. They are also singly refracting and hence exhibit no dichroism and therefore lack variety of colour as compared with true ruby. Some especially fine ones, however, are of a good enough red to deceive even jewelers of experience, and one in particular that I have in mind has been the rounds of the stores and has never been pronounced a spinel, although several "experts" have insisted that it was a created ruby. The use of a dichroscope would have saved them that error, for the stone is singly refracting. Spinels are usually clearer and more transparent than garnets and show their colour better at a distance or when in a poor light.
Tourmaline of the reddish variety (rubellite) is seldom of a deep red. It is more inclined to be pinkish. The dichroism of tourmaline is stronger than that of ruby and more obvious to the unaided eye. The red of the rubellite should not deceive anyone who has ever seen a fine corundum ruby.
The species that furnish blue stones in sufficient number to deserve consideration are, aside from opaque stones:
Of these minerals the only species that furnishes a fine, deep velvety blue stone is the corundum, and fine specimens of the cornflower blue variety are very much in demand and command high prices. The colour in sapphires ranges from a pale watery blue through deeper shades (often tinged with green) to the rich velvety cornflower blue that is so much in demand, and on to dark inky blues that seem almost black by artificial light. Most sapphires are better daylight stones than evening stones. Some of the sapphires from Montana, however, are of a bright electric blue that is very striking and brilliant by artificial light.
The direction in which the stone is cut helps determine the quality of the blue colour, as the "ordinary" ray (sapphire exhibits dichroism) is yellowish and ugly in colour, and if allowed to be conspicuous in the cut stone, its presence, blending with the blue, may give it an undesirable greenish cast. Sapphires should usually be cut so that the table of the finished stone is perpendicular to the principal optical axis of the crystal. Another way of expressing this fact is that the table should cross the long axis of the usual hexagonal crystal of sapphire, at right angles. This scheme of cutting puts the direction of single refraction up and down the finished stone, and leaves the ugly ordinary rays in poor position to emerge as the light that falls upon the girdle edges cannot enter and cross the stone to any extent.
A dichroscope may be sued to find out whether a lapidary has cut the finished stone properly as regards its optical properties and if there is little or no dichroism in evidence when looking through the table of the stone it is properly cut.
Where a sapphire shows a poor colour and the dichroscope shows that the table was laid improperly, there is some possibility of improving the colour by recutting to the above indicated position. However, judgment must be used in such a case, as sapphires, like other corundum gems, frequently have their colour irregularly distributed, and the skillful lapidary will place the culet of the stone in a bit of good colour, and thus make the whole stone appear to better advantage. It would not do to alter such an arrangement, as a poorer rather than better colour may eventuate by recutting in such a case.
While some of the blue stones about to be described may resemble inferior sapphires, none of them approaches the better grades of sapphire in fineness of blue colouration. The created sapphire, of course, does approach and even equals the natural sapphire so it is necessary to know how to distinguish between them. This distinction is not one of colour, however, and it will be separately considered a little later.
Blue spinels are infrequently seen in commerce. They never equal the fine sapphire in their colour, being more steely. They, of course, lack dichroism and are softer than sapphire as well as lighter.
Blue tourmalines are never of fine sapphire blue. The name indicolite which mineralogists give to these blue stones suggests the indigo-blue colour which they present. The marked dichroism of tourmaline will also help detect it. Some tourmalines from Brazil are of a lighter shade of blue and are sometimes called "Brazilian sapphires."
Blue topaz is usually of a pale sky blue or greenish blue and is likely to be confused with beryl of similar colour. The high density of topaz (3.53) as compared with beryl (2.74) serves best to distinguish it.
Blue diamonds are usually of very pale bluish or violet tint. A few deeper blue stones are seen occasionally as "fancy" diamonds. These are seldom as deep blue as pale sapphires. Even the famous Hope Blue Diamond, a stone of about forty-four carats and of great value, is said to be too light in colour to be considered a fine sapphire blue. Some of the deeper blue diamonds have a steely cast. The so-called blue-white stones are rarely blue in their body colour, but rather are so nearly white that the blue parts of the spectra which they produce are very much in evidence, thus causing them to face up blue. There is little likelihood of mistaking a bluish diamond for any other stone on account of the "fire" and the adamantine luster of the diamond.
Blue zircon, however, has nearly adamantine luster and considerable fire. The colour is usually sky blue.
Pink Stones. The species that furnish pink stones are:
These pink minerals are not easily differentiated by colour alone, as the depth and quality of the pink vary greatly in different specimens of the same mineral and in the different minerals. Pink diamonds, such as some being discovered in the Argyle mines of Australia, can be distinguished from other pink stones by their adamantine luster, and their prismatic play or "fire." There is dichroism in the cases of pink sapphire, pink tourmaline (strong), pink topaz (strong), pink beryl (less pronounced), and kunzite (very marked and with a yellowish tint in some directions that contrasts with the beautiful violet tint in another direction in the crystal). Pink quartz is almost always milky, and shows little dichroism. Pink spinel is without dichroism, being singly refracting. Hardness and specific gravity tests will best serve to distinguish pink stones from each other. The colour alone is not a safe guide.
Purple Stones. Among the mineral species that furnish purple stones are:
The purple of the amethyst varies from the palest tints to the full rich velvety grape-purple of the so-called Siberian amethysts. The latter are of a reddish purple (sometimes almost red) by artificial light, but of a fine violet by daylight. No other purple stone approaches them in fineness of colouring, so that here we have a real distinction based on colour alone. If the purple is paler, however, you cannot be sure of the mineral by its colour. Purple corundum is seldom as fine in colour as ordinary amethyst, and never as fine as the best amethyst. It is usually of a redder purple, and by artificial light is almost ruby-like in its colour.
Purple spinels are singly refracting, and lack dichroism, and hence lack variety of colour.
Almandine garnets also show no dichroism and lack variety of colour. The garnets are, as a rule, apt to be more dense in colour than the spinels.
Purple spodumene (kunzite) is pinkish to lilac in shade—usually pale, unless in large masses, and it shows very marked dichroism. A yellowish cast of colour may be seen in certain directions in it also, which will aid in distinguishing it from other purple stones.
Brown Stones. Among the mineral species that furnish the principal brown stones are:
Diamond, when brown, unless of a deep and pleasing colour, is very undesirable, as it absorbs much light, and appears dirty by daylight and dark and sleepy by artificial light. When of a fine golden brown, such as some being discovered in the Argyle mines of Australia, a diamond may have considerable value as a "fancy" stone. Such "golden fancies" can be distinguished from other brown stones (except perhaps brown zircons) by their adamantine luster, and their prismatic play or "fire."
Brown garnet (hessonite) is of a deep reddish-brown colour. Usually the interior structure, as seen under a lens, is streaky, having a sort of mixed oil and water appearance.
Brown tourmaline is sometimes very pleasing in colour. It is deep in shade, less red than cinnamon stone, and with marked dichroism, which both brown diamond and brown garnet lack.
Brown zircon, while lacking dichroism, is frequently rich and pleasing in shade, and when well cut is very snappy, the luster being almost adamantine, the dispersion being large, and the refractive index high. It is useless to deny that by the unaided eye you might be deceived into thinking that a fine brown zircon was a brown diamond. However, the large double refraction of the zircon easily distinguishes it from diamond (use the sunlight-card method or look for the doubling of the edges of the rear facets as seen through the table). The relative softness (7.5) also easily differentiates it from diamond.
Few colourless stones other than diamond, white sapphire (chiefly created), and quartz are seen in the trade. Colourless true topaz is sometimes sold and artificially whitened zircon is also occasionally met with. Beryl of very light green tint or even entirely colourless may also be seen at times.
Such colourless stones must of course be distinguished by properties other than colour. This does not except the diamond, which is rarely truly colourless.