LESSON XIII
COLOR—Continued
BLUE STONES
The species that furnish blue stones in sufficient number to deserve consideration are, aside from opaque stones:
1. Corundum (sapphire).
2. Spinel.
3. Tourmaline.
4. Topaz.
5. Diamond.
6. Zircon.
1. 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 color 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.
How Sapphires should be Cut. The direction in which the stone is cut helps determine the quality of the blue color, as the "ordinary" ray (sapphire exhibits dichroism) is yellowish and ugly in color, 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.
To find out with a finished stone whether or not the lapidary has cut it properly as regards its optical properties one may use the dichroscope, 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 color and the dichroscope shows that the table was laid improperly, there is some possibility of improving the color by recutting to the above indicated position. However, one must use much judgment in such a case, as sapphires, like other corundum gems, frequently have their color irregularly distributed, and the skillful lapidary will place the culet of the stone in a bit of good color, and thus make the whole stone appear to better advantage. It would not do to alter such an arrangement, as one would get poorer rather than better color 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 coloration. The scientific sapphire, of course, does approach and even equals the natural sapphire so that one must know how to distinguish between them. This distinction is not one of color, however, and it will be separately considered a little later.
2. Blue spinels are infrequently seen in commerce. They never equal the fine sapphire in their color, being more steely. They, of course, lack dichroism and are softer than sapphire as well as lighter.
3. Blue tourmalines are never of fine sapphire blue. The name indicolite which mineralogists give to these blue stones suggests the indigo-blue color which they afford. 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."
4. Blue topaz is usually of a pale sky blue or greenish blue and is likely to be confused with beryl of similar color. The high density of topaz (3.53) as compared with beryl (2.74) serves best to distinguish it.
"Fancy" Blue Diamonds. 5. 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 color 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 color, 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.
6. Blue zircon, however, has nearly adamantine luster and considerable fire. The color is usually sky blue. Such stones are seldom met with in the trade.
For a more detailed account of the various blue stones see G. F. Herbert-Smith's Gem-Stones, as follows:
For sapphires, pp. 172-173, 176, 182; for spinel, pp. 203, 204, 205; for tourmaline, pp. 220, 221, 223; for topaz, pp. 198, 200, 201; for diamond, pp. 130, 136, 170, and for zircon, pp. 229, 231.