Corundum is one of the fairly common minerals in the earth's crust. It is found in many different kinds of rock and in many places on the earth's surface, but only under the rare occurrence of nearly ideal conditions is transparent material formed. With the exception of black star sapphire, only the transparent to translucent varieties are classed as gemstones.
Ruby
Only transparent corundum of medium light to dark tones of red to purple-red hues is properly called ruby. Very light tones of red are correctly called pink sapphire. Even some stones that are light rather than very light in tone are properly called pink sapphire. A very intense red color is necessary to justify the use of the term ruby.
On the basis of the usual product of certain ruby-producing countries, several terms are used in the American jewelry industry to describe different colors of ruby. There is too little consistency in trade grades to suggest that any stones meeting these descriptions will always, or even almost always, be described by the appropriate term. The trade grades listed below are included so that the student will understand their usual meaning, rather than as a recommendation that they be employed.
Burma, Burmese or Oriental Ruby
The finest blood-red rubies ("pigeon's-blood" ruby is the term often applied to the finest) are called Burma rubies, despite the fact that some very dark and some pale-colored stones also come from Burma. In some cases, the word Oriental is used to describe a ruby of fine quality; but since all three of the major ruby sources are in the Orient, the use of this term is more confusing than helpful. In Europe, it tends to be used for stones that are called Burma rubies in this country. Two somewhat less desirable subdivisions of this fine color are "beef-blood" rubies (slightly darker tones of red) and "French-color" or "cherry" rubies (slightly lighter than a so-called blood red).
There is no doubt that the finest rubies are found almost exclusively in Burma; however, there are also Burmese rubies that are lower in quality than some of those found elsewhere. If terms such as Burma ruby are to be used for trade: grades, they should be applied only on the basis of quality, regardless: of source. Thus, even though a stone comes from Burma, if it is light in tone and more nearly a pink sapphire than a ruby, it should not be called a Burma ruby, regardless of its origin. Some rubies from Thailand and some from Tanzania have the beauty associated with the good Burma qualities.
Siam, or Siamese, Ruby
This term is still used, despite the fact that the country of origin is now called Thailand. The usual output from the Thailand mines is dark red to brownish red, sometimes resembling almandite or pyrope garnet. These stones are rarely of a quality that even approaches that of fine Burma stones. They are distinctly less desirable. However, not all Thai rubies are dark or brownish. Some from nearby Pa Hill, Cambodia, and also from other mines in Thailand are bright and very desirable.
Ceylon Ruby
Light to very light red or purplish-red stones are mined in Sri Lanka, formerly Ceylon. Those that are medium light in tone (i.e., not light to very light), so that they may appropriately be classed as ruby, are called Ceylon rubies. The light to very light red, of course, should be called pink sapphire. Because of their lighter tone, these stones are usually more brilliant than Burmese or Siamese stones. Fine stones from Sri Lanka usually bring higher prices than the better qualities from Thailand.
African Ruby
Some good quality purplish-red (pigeon's-blood) faceting grade ruby occurs at Longido and Lossogonoi, Tanzania; however, the material appears to be sheet like and melee size stones are the major production. Some excellent quality ruby of large size, up to 8 carats faceted, has come out of Tsavo National Park, Kenya. Good cabochon ruby is also reportedly found in northern Kenya along the Tana River.
Star Ruby
Stones that have a ruby color and that also display a distinct asterism when cut in cabochon are called star rubies. The tendency is to allow more latitude in color when applying the term ruby to stars than is true with faceted stones. In other words, stones that do not have a true ruby color (e.g. those that are too pale or too purple) are sometimes passed off as star rubies. The only justification for a difference in color stands in this regard is that standards the "silk" in asteriated stones makes them slightly less transparent and tends to gray the color and lighten it. A cat's-eye effect has also been encountered in ruby but it is exceedingly rare, especially in a sharply defined band.
Sapphire
Since the term sapphire is applied accurately to all colors of corundum other than medium light to dark tones of red to purple-red it is best to use a color prefix for the various colors of sapphire other than blue. Sapphire used alone means the blue variety to laymen and jewelers alike. Blue sapphire is also divided into trade grades in the jewelry industry, but these grades are not as clearly based on the usual output or the better quality output of the important present or former sources of sapphires, as are the trade grades of ruby. Since there are more sapphire mines producing fine qualities than is true of ruby mines, the locality names associated with sapphire trade grades are less meaningful and less consistent. The trade grades of blue sapphire are as follows:
Kashmir or Cashmere Sapphire
Kashmir sapphires are encountered today, even though the mines in that area are not presently being worked, nor have they been worked for a number of years. The term is applied to velvety violetish-blue stones (so-called cornflower blue) that are not exceedingly transparent; this lack of transparency produces a "sleepy" appearance, unlike that of other blue sapphires. Although the term Kashmir has persisted for many years and the implication is that such sapphires are not longer found, this is not true, since occasional stones mined in Burma and Thailand have the same appearance as the Kashmir grade. Actually, a significant percentage of the product of the Kashmir mines, when they were operated, was paler but more brilliant than the finest stones. Magnificent sapphires are found in Montana, Thailand and even in Australia, so trade grades need have no bearing on source.
Burma or Oriental Sapphire
This term is used in the American trade for a very fine quality. It has been described as a fine "rich" blue or "royal' blue (slightly violetish blue). The distinction between the Burma stones lose some of their color and appears somewhat inky under artificial light.
Siam or Siamese Sapphire.
This term is used to describe very dark blue stones. Since they seem to be exceedingly dark, even in daylight, they might appropriately be called almost blue-black in color. In England, the term Siam sapphire has been used to indicate a stone ranking next to Kashmir in grade: an intense dark blue with a slightly velvety body appearance. Although it is not used for that type of stone in American, the description does cover some of the fine stones mined in Thailand. The American trade-grade description, however, applies to a poor grade.
Ceylon Sapphire
This term is usually applied to pale grayish-blue to light blue-violet and fairly brilliant stones. Some stones contain a considerable amount of silk, which reduces brilliancy and may impart a grayish color. Blue Ceylon sapphires are somewhat subject to unevenness of color. Fine sapphires from Sri Lanka are among the finest.
Montana Sapphire
Since the two producing regions in Montana (described under Sources and Recovery Methods) yield, or yielded, distinctly different types of sapphires, the term Montana has little relevancy. It actually described the alluvial production more accurately than the output from in place deposits in Yogo Gulch. The alluvial production is mostly light in color. The blue stones have been described as an electric or 'steel' blue and were highly transparent. Such stones are said to have a "metallic" bluster, which means little or nothing, except that they perhaps give the impression of a higher luster than the; softer appearing, velvety blues from other sources. The term "new-mine" sapphire, which is heard occasionally in the trade, is derived from the New Mine Syndicate, the British concern that owned and operated the Yoqo property prior to 1929. This term was applied to the darker stones among those produced from that deposit.
African Sapphire
African sapphires occur in a variety of pastel colors: pale blue, blue violet, red violet, pale yellow, pale orange, steel-gray and dark brownish orange from the Umba gravels. Some African sapphires exhibit an alexandrite like color change, especially from the Umba River area where some change from steel-blue to a green or to purple. Various colors of pale sapphires and rubies are also found in southern Malawi.
Australian Sapphire
Stones from Australia are commonly very dark and inky and are usually characterized by strong green to very dark violet blue dichroism. Stones of this grade are not necessarily confined, to Australia, but are found also in Siam and elsewhere. Dark feathers and strong color zoning are common in Australian corundum.
Fancy Sapphire
Most of the stones other than blue are called fancy sapphires; however, they are often referred to simply as green sapphire, yellow sapphire, etc. Almost all of the colors other than blue are represented. The majority of those seen in the trade originate in Ceylon.
Yellow sapphire
Yellow sapphire, which is usually referred to as golden sapphire, is a fairly important gemstone. It usually occurs in slightly brownish-yellow colors in light to medium-dark tones. Intense golden yellow, which is rather highly prized, was once referred to Widely as "Oriental topaz", "King topaz" or "imperial topaz".
Intense Orange to Orange-Red
Among the fancy sapphires, stones of these colors are rare and highly prized and considered by many connoisseurs to be among the most beautiful of all gemstones. The intense light reddish orange stones are often called padparadscha (pad-par-AHD-shah) or padparadscha sapphires. The color resembles that of the lotus. Note that dark brownish-orange stones from the Umba River in Tanzania should not be called padparadscha. The orange-red stones are sometimes called bhyacinth sapphires. Both colors find willing buyers in Ceylon itself and these stones rarely enter the foreign trade.
Green Sapphire
Most of the sapphires sold as green are inky stones of the blue Australian or Siamese types that have been cut to display the green rather than the dark-blue dichroic color. When a lighter green stone occurs, it is very attractive and has a certain amount of demand; however, it is too uncommon to be an important stones. It has often been incorrectly called "Oriental emerald" although the color can hardly be compared to that of emerald.
Violet to Purplish Violet
Stones that have a violet or purplish-violet color are usually referred to as amethystine sapphire or, incorrectly, as "Oriental amethyst". Reddish-purpose stones are more frequency called plum sapphires, rather than amethystine sapphires. Some dealers refer to plum-colored stones as "rubies".
Pink, or Rose Sapphire
Light-red sapphires (i.e. stones that are too light in tone to be called ruby) are usually referred to as pink sapphire. Some of them are very attractive and in demand. A problem in classification frequently arises with stones that appear red under incandescent light and light purplish-red in daylight. In order to avoid future difficulties in selling, it is recommended that such stones should not be called rubies, unless they are medium red under both light sources.
Alexandrite like Sapphire
Some sapphires show a distinct change of color from violet or purple to blue. Stones that change to a more reddish or purplish color at night from a daylight blue color are usually referred to as alexandrite like. The color change is usually weak and detracts from, rather than adds to, beauty. However, rare stones show a very strong change from an attractive daylight blue to a lovely purple-violet night color.
Colorless Sapphire
Colorless sapphires were used for many years as diamond substitutes, particularly in calibre sizes for jewelry pieces other than rings. They are usually referred to as white sapphires or rarely, as leuco sapphires. Natural colorless sapphires are seldom as transparent or as brilliant as synthetic colorless sapphires or synthetic spinels, which have replaced them for calibre use.
Light Greenish Blue
Sapphires of this color are often referred to as aquamarine sapphires or sometimes, in error, as "Oriental aquamarine".
Brown Sapphire
Transparent brown sapphires are rather rare; usually, they are opaque. If they are sufficiently silky, cabochon-cut stones may exhibit a star; if not, a term sometimes used for them is adamantine spare. Attractive transparent stones of a moderately intense brown color have been found in Cambodia.
Star Sapphire
Any sapphire that shows asterism is referred to as star sapphire. These stones are never completely transparent, because they must have enough silky, needlelike inclusions to produce a star. They occur in almost every color in which transparent sapphires exist, although orange and yellow stars are almost unknown and green is somewhat less rare. Blue ones range from a rich, deep blue , which is rare in stones with distinct stars, to those containing so little blue that they are bluish gray to almost white.
The other frequently encountered asteriated variety is the black star sapphire, so called despite the actual very dark brown, purple, blue or green color. Most black stars show strong zonal and inclusion distribution, in addition to basal twinning, which makes them subject to easy parting. As a consequence, the majority must be cut in very flat cabochons, which makes the zoning more obvious and reduces the sharpness of the star. Twinned stones frequently display a twelve-rayed star, the alternate legs of which vary in intensity and occasionally in color.
Unequal distribution of inclusions may weaken one or more, rays of a star ruby or sapphire, producing a cat's-eye effect. Also, this effect may result if the optic axis of a star stone is oriented parallel to the girdle. These are curiosities, rather than significant gem varieties, and are seldom encountered, they are, however, greatly esteemed by the gem connoisseur.
