Gemstones

Before the dawn of history people were finding and assigning special values to certain kinds of rock and mineral pebbles that were rare or particularly beautiful.  Thus was the birth of the gemstone, a mineral set apart from the rest by beauty, rareness and durability, which is a relatively uncommon combination of attributes.

Gemstone lore, legend and superstition have been part of every human society. In almost every culture gem materials have been given religious or spiritual significance. The signs of the zodiac, for example, have been associated in several religions with gemstones that traditionally helped them to exercise their influence over mortals.

At the beginning of the fifth century, St. Jerome spoke of the relationship between the twelve gemstones in the breastplate of the Jewish High Priest, the twelve months of the year, and the twelve signs of the zodiac. However, the custom of wearing a birth-month stone is not ancient. It seems to have developed in Poland in the eighteenth century and is perpetrated to this day in the official birthstone list of the jewelry industry. The garnet belongs to January, February is symbolized by the amethyst, the aquamarine or bloodstone for March, April has the diamond, the emerald in May, June is represented by the moonstone or pearl, the ruby is granted to July, in August it is the peridot or sardonyx, the sapphire belongs to September, October is represented by the opal or tourmaline, the topaz or citrine symbolizes November, and the turquoise or lapis lazuli belongs to December.

The occult powers of gems may be very much open to interpretation, but they have besides beauty and durability a concrete characteristic that makes them highly desirable. Gems combine high value with small size. They are easy to transport, easy to conceal, and easily convertible into money. In times of stress, when the value of money has weakened, gems have often been the means of saving the substance of an estate. Real estate holdings may not survive a revolution, but a secret hoard of gems may outlast any political system.

With expanding populations and increasing prosperity in several countries of the world, the number of potential customers for gems is increasing far more rapidly than the available supply. The result of this is a vast annual trade and rising prices. Unfortunately for the buying public, it generally does not know much about gemstones.

It is not essential for the gem buyer to master truly technical information, but there are certain basic facts that can be easily assimilated and can reinforce a consumer with a certain amount of knowledge even though they must ultimately rely on the jeweler in technical matters. Certain essential characteristics must always be reviewed when the quality and value of a gemstone are under consideration.

The hardness of a gem is the resistance it gives to scratching and general wear. Moh's Scale, a rough scale used for all minerals, is the convenient and generally accepted guide.

All gemstones of any importance have a hardness above 6 in this scale. If less than 6 in hardness, they are not durable enough for use as gems. Any gem material that is not scratched by a sample of quartz, but is by quartz would have to have a hardness of 7.5. For testing purposes and to help identify a gemstone by its hardness, a jeweler may use a set of pencil-like tools with hardness points made of minerals in the scale.

The weight of gemstones is usually given in carats where one carats equals one-fifth of a gram. Converting this into more familiar units, there are about 140 carats in an ounce. Even knowing that the carat is an expression of weight and not size, gem buyers are always surprised to see that a 1 carat sapphire is considerably smaller than a 1 carat diamond. Sapphire is denser than diamond, and a smaller stone can therefore weigh the same number of carats as a larger diamond.

Determining the specific gravity of a gem involves measuring the size and weight at the same time. We are aware of the difference in weight when we compare iron and wood, yet it would not always be correct to say that iron weighs more than wood since a larger piece of wood can weigh more than a small piece of iron. Only by comparing equal volumes of these materials can the extent of the weight difference be clear and unmistakable. Diamond is three and one-half times heavier than an equal volume of water (which is used as a standard in specific gravity determinations), hence the diamond's specific gravity is 3.5.

The ability of mineral structures to bend a beam of light easily be demonstrated. The amount of bending or refraction depends entirely on the species of mineral involved, because it is an effect brought about by the mineral's crystalline structure. The degree of bending, measured by a refractometer, is useful in gem identification since every gem has a different index of refraction.

When Newton split a beam of white light into various wavelengths, he demonstrated the phenomenon of dispersion. The amount of refraction when a beam of light strikes a gemstone depends not only on the structure but also upon the wavelength of the light. The blue wavelength, for example is bent more than the red. Since white light is composed of all wavelengths, each tends to be bent a different amount, and the light is thus separated and sorted into a spectrum of colors. If the gemstone has strong dispersive ability, like diamond, it will cause a wider separation of the wavelengths and sparkle with colored flashes whenever struck by a beam of white light.

Some effects are the result of inclusions or foreign bodies found in some mineral crystals. Generally inclusions are considered undesirable in gemstones except as an aid in identification. However, some inclusions can greatly enhance the value and beauty of a gem by producing striking light effects.

Chatoyant mineral specimens contain inclusions of needle-shaped reflecting bodies of other species, such as rutile. These inclusions may even be hollow tubes. A combined reflection phenomenon called chatoyancy is produced when the tubes or needles are oriented in parallel bundles. By cutting the gemstones as oval or circular domes, the gem cutter is able to reveal the resemblance of a cat's eye.

Asterism is an effect that can occur when there are several sets of needle inclusions. In crystals of corundum, from which stem rubies and sapphires, these sets arrange themselves in three different directions 120 degrees apart, parallel to the possible crystal faces. Instead of one band of light there are now three bands intersecting at a common point. If the gem is properly cut the effect is a reflection of a bright star of light.