Bridal Sets

Star stones are sometimes seen, but star rubies are much rarer than star sapphires. These stones that can be used as bridal sets, which are not faceted but cut en cabochon, display a six rayed star on their summit. In poor specimens, which are in the majority, the star is rather indistinct and the color of the stone is pale. This star effect, known as asterism, is due to naturally included minute cavities which are arranged parallel to the six sides of the prism, and which reflect the incident light. Another and more frequent cause of this phenomenon is the mineral rutile. This, in the form of a great number of very thin needles which have arranged themselves parallel to the prism faces of the hexagonal crystal in its forma­tion, reflects light to produce a star. A similar effect is occasionally seen in some forms of quartz, almandine garnet, and very rarely emerald, but stones belonging to the cubic system would naturally show a four rayed star. Garnets also have 6-rayed stars.

On very rare occasions, also, sapphires and rubies, as well as tourmalines, show chatoyancy, an effect generally associated only with chrysoberyl and quartz cat's-eye.

The rarest and most expensive shades are the so-called "pigeon blood" red with rubies, and the "cornflower" blue with sapphires. Large and perfect stones, particularly rubies, are very scarce and, despite all synthetics and imitations, reach a very high price in the market. Such bridal sets are very very popular. A perfect 5 carat ruby has been known to realize $2,800 to $4,000 per carat, but flawless stones of this size and quality are very rare. Most of the cut stones will be seen to con­tain faults, and sometimes silky-looking internal patches. This "silk" is due to an area of parallel minute canals from which light is reflected, but in some rubies, particularly those from Thailand (Siam), this effect is thought to be caused by the inclu­sion of very minute crystals of titanic oxide. Most of the Siamese stones possess these inclusions, which do not, however, produce asterism.



Darker specimens always contain a percentage of iron oxide. Yellow tints are caused by ferric oxide, and the blue sapphire con­tains a percentage of titanium.

Cut stones are seen in brilliant, mixed, and step forms, the mixed cut being the most common; star stones are always cut en cabochon. Ceylon sapphires are generally cut steeply so that all emerging light passes through the most deeply colored part of the stone, which is usually at the base. The native lapidaries of Ceylon are adept in producing the best effects from poorer stones, but such specimens are often difficult to mount in jewelry since they are thick, and they are therefore often re-cut.



The color of rubies and sapphires varies with locality, and dealers in colored gem stones can generally say where a given specimen has been mined, although color alone is not an infalli­ble guide. Siamese stones are generally darker than the Burmese; those from Ceylon are generally pale and parti-colored, and prices vary accordingly, the Burmese stones being the most expensive although they almost always contain patches of "silk." Siamese stones are not characterized by this defect but more often by in­ternal feathers or flaws of a brownish color, which are caused by iron compounds. Ceylon stones also contain feathers, and they are generally of the cheapest qualities. A large Ceylon sapphire, weighing as much as 42 lbs. in the rough, was cut in London dur­ing 1936, but its quality was such that it was quite useless for jewelry purposes. It should be noted that sapphires are best examined for color in a good natural light; in artificial light, some of the blue stones take on a lavender tint so specimens should be doubly checked.



There is very little "fire" in rubies or sapphires since dispersion is small. Thus white sapphires look dull against a diamond, but many other properties would distinguish between these stones. The grade of hardness is next to diamond, being recognized as 9 on the Mohs' scale, but in this respect corundum falls far below the diamond and it is relatively much softer than this degree suggests. Hardness is not absolutely constant, for rubies are gen­erally slightly softer than sapphires. Also Ceylon sapphires are harder than the rare Cashmere sapphires. Crystals break evenly along the parting planes, so they should be handled with care. As for density, this ranges from 3.99 to 4.00, and most stones fall within these narrow limits. Burmese and Ceylon rubies have a slightly lower density than those from Thailand, and it also in­creases slightly with depth of color. Green sapphires are usually the heaviest of the corundums.

The many colored varieties all show dichroism, a property Which must be considered by the lapidary when cutting crystals.

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