Of all the color-varieties of precious corundum the red, or ruby ("oriental ruby"), is the most highly prized. It is probably identical with the anthrax of Theuphrastus, and is one of the stones referred to in medieval times as carbuncle. It has all the general characters of corundum, and is only distinguished from other varieties by its' red color.
The tone of color differs in different specimens, being sometimes deep and intense ("masculine" ruby), sometimes pale and light ("feminine" ruby). The lighter shades vary from pale rose-red to reddish-white, some specimens being so faintly tinged with red as to appear almost colorless. The darker colors are pure red, carmine-red, or blood red; the red of the majority of rubies, however, has a more or less distinct tinge of blue or violet, this being especially noticeable in transmitted light. The shade of color which is most admired is the deep, pure carmine-red, or carmine-red with a slight bluish tinge. Burmese has compared this color to that of the blood of a freshly-killed pigeon, hence the references to such stones as being of "pigeon's-blood" red. The various shades of red of the ruby are remarkable in that they lose none of their beauty in artificial light, a statement, which cannot be made respecting any other precious stone of the same color.
The coloring of rubies is not always perfectly uniform, colorless layers being sometimes interposed between portions colored red. In such cases, the stone will often become uniformly colored throughout after heating. Provided the stone is gradually heated it may be raised to the highest temperatures with no fear of fracture. The interesting changes in color exhibited by certain gems when gradually heated and then allowed to cool have been already described. During cooling the ruby becomes first white and then green, finally regaining its original red color, so that in this stone the coloring matter is neither permanently changed nor destroyed by exposure to high temperatures. It is otherwise, however, with the sapphire, for this gem at a high temperature loses its beautiful blue and takes on a dull gray color. The red coloring-matter of the ruby is therefore certainly not organic in nature, as seems to be the case with those gems, which lose their color on heating. It is more likely to be some compound of chromium, an element whose presence has been detected in the analysis of some rubies. That the coloring of the ruby is due to chromium is also suggested by the fact that the color of the so-called "ruby" glass is obtained by adding a small amount of chromium oxide to the other constituents of the glass. M. Fremy also used the same substance for the production of the red color of his artificially prepared rubies. Some of the crystals produced by this investigator were partly red and partly blue, resembling in this respect certain natural rubies, which occur rarely in Burma.
The dichroïsm of deeply colored rubies is very noticeable; with the exception of stones of very pale color, a difference in the color; of every ruby can be observed when viewed in different directions. On looking through a dark-colored crystal of ruby, such as is illustrated in Fig. a-d, in a direction perpendicular to the basal planes, it will appear of an intense red color, either pure red or with a slight tinge of violet. If, however, the light received by the eye has passed through the crystal in a direction perpendicular to a prism face or edge, the stone will appear much lighter in color. On allowing the light, which has passed through the crystal in this direction to enter the dichroscope, the two images, in that position of the instrument in which the greatest difference in color is shown, will be one light, and the other dark red usually tinged with violet. In all other directions in which the light may travel, with one exception, the two images will be more or less differently colored. This exceptional direction is perpendicular to the basal planes and coincides with the direction of the optic axis. Along this direction the crystal is singly refracting, and the two images seen in the dichroscope are of the same deep red color as the crystal appears when viewed in this direction without the intervention of the dichroscope. The dichroïsm of the ruby affords a means whereby it may be distinguished with certainty from other red stones, such as spinel and the different varieties of garnet, which crystallize in the cubic system, and thus being singly refracting can show no dichroïsm.
The fact that the color of the ruby varies with the direction in which it is viewed, makes it necessary that the form of the cut gem should have a certain definite relation to that of the crystal in order to obtain the finest color-effect. The plane of the largest facet of the cut stone, namely, the table, must coincide as closely as possible in direction with the basal planes of the crystal in order to obtain the greatest depth in color of which the stone is capable. The greater the angle at which the table is inclined to the basal plane of the crystal the poorer will be the color-effect produced, and when the table is perpendicular to the basal plane, and therefore parallel to the prism faces of the crystal, the minimum color-effect is the result.
Some rubies show on the basal plane, or still more plainly on a cut and polished curved surface approximating to the basal plane in direction, a six-rayed star of glimmering reflected light. Such stones are known as star-rubies, or asteriated rubies, sometimes also as ruby cat's-eye. The appearance is similar to that seen in the star-sapphire, but, as a rule, less marked; it will be therefore considered in greater detail under sapphire.
A clear, transparent, and faultless ruby of a uniform deep red color is at the present time the most valuable precious stone known. Except in ancient times, it is probable that the ruby has always held a foremost place in the estimation of connoisseurs. This, however, is not hue of stones of a pale red color, which are always less highly prized, on account both of their light shade of color and of the fact that they occur more abundantly and in larger size than do stones of a true pigeon's-blood red.
The value of the finest ruby, therefore, far exceeds that of a diamond of corresponding size and quality. One-carat diamonds of the first water are of far more frequent occurrence than rubies of the same size and quality, while large rubies are still more rare than large diamonds. A fine deeply colored ruby of 3 carats is a great rarity, whereas it is by no means unusual to come across one diamonds of this size. Again, while 10-carat diamonds are of moderately frequent occurrence, rubies of the same weight scarcely ever occur, while only very few specimens of still larger stones are known. It is therefore to be expected that larger rubies should command exceptionally high prices; indeed, the prices of stones of ordinary sizes may be arrived at very closely by the application of Tavernier's rule. The relation between the ratio of the weight and the value of a large and a small stone is very different when, on the one hand, the two stones are diamonds, and when, on the other, they are rubies. Thus, while a 1-carat ruby is worth twice as much as a 1-carat diamond, a 3-carat ruby of the first quality is worth ten times as much as a diamond of the same description, that is to say, that while a 3-carat brilliant of the first water would be valued at about $16,300, a ruby of the same description would be worth about $160,300. The value of a 5-carat diamond of the first quality would be about $33,000, while that of a similar ruby would be $330,000. These values, of course, apply to stones in the cut condition, the weight of which uncut would be about doubled. For rubies of still larger size there is no fixed market price; almost fabulous sums have been paid for very fine stones of large size required for some special purpose. A fine ruby of 9 5/l6 carats was valued by Mr. G. F. Kunz, the American gem expert, at $736,360. Again, $1,086,550 was stated by Mr. E. W. Streeter, the London jeweler, to have been paid for a cut ruby of 32 5/16 carats, and double this amount for another weighing 38 9/16 carats; both of these stones were faultless specimens of magnificent color.
It is recorded by Benvenuto Cellini in the middle of the sixteenth century that a carat ruby was eight times the value of a carat diamond, the price of the former being 800 golden scudi ($17,400) and that of the latter 100 scudi ($2,175). The ratio at the present time is only about two to one, the market price of a fine 1-carat ruby being about $2,750, and that of a brilliant of the same weight about $1,100, only in very exceptional cases $2,200 or $2,720 being paid. The particular shade of color shown by a ruby exercises an enormous influence on its value, thus a carat stone of a pale rose color is worth at the most but $110, which contrasts strangely with the value of a stone of equal size, but of a deep red color.
The value of any particular ruby does not reach the high figures mentioned above unless it is an absolutely faultless specimen. The faults most commonly met with are lack of clearness; existence of cloudy portions (so-called "clouds"), especially frequent in light colored stones; milk-like, semi-transparent patches ("chalcedony patches"); small internal cracks and fissures ("feathers"); unequal distribution of color, and so on.
Just as some few diamonds, on account of their singular beauty, large size, or unique color, have become famous and well known, so certain rubies on account of their exceptional size have acquired more or less fame and renown. Tavernier states that he saw two rubies, in the possession of the King of Bijapur, in India, which weighed 50 3/4 and 17 1/2 carats, and which he valued at 600,000 ($2,607,750) and 74,550 francs ($324,000) respectively. Other large rubies have been occasionally met with in India and especially in Burma. The King of Ava was reported to be in the possession of a ruby mounted as an ear-pendant, which is the size of a small hen's egg. A few specimens of similar exceptional size are known in Europe. Kaiser Rudolph II of Germany possessed a ruby of flawless beauty and of the size of a hen's egg, which was valued by the gem expert Boetius de Boot at 60,000 ducats (about $3,042,350). It is related that in 1777 Gustavus III of Sweden presented to Catharine II of Russia a beautiful ruby of the size of a pigeon's egg; the present whereabouts of this stone is, however, unknown. The largest of the fine rubies set in the French crown, according to the inventory made in 1791, weighed 7 carats, and was then valued at 8,000 francs ($34,770).
Another weighed 25 11/16 carats, but on account of its pale color was valued at no more than 25,000 francs ($109,660). Other rubies of large size will be mentioned under the description of localities. The largest ruby known is said to be from Tibet; it weighs 2,000 carats, but is not perfectly transparent. The largest ruby yet found in Burma is also a little cloudy; Streeter gave its weight at 1,184 carats. Professor John Ruskin presented a tabular crystal of ruby of a rich red color, and in part perfectly transparent, to the British Museum; it had a weight of 162 2/3 carats.
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