There are several ways to select a gem. Starting with how the gem will be used, such as in a ring setting or in a bracelet or a necklace, the hardness of a gem is most important. Moe's Scale of Hardness is a useful starting place. This scale has ten hardness levels from the diamond on down to the very soft soapstone. Stones which are naturally hard are durable in such settings as rings. Rings get the most exposure to contact with hard surfaces, either with regular wear or accidental contact. Stones high on the scale of hardness include sapphire, spinel, and of course diamond. For other settings such as a necklace, any stone could be considered, including the soft but popular Tanzanite. Bracelets can get hard wear but for the most part, the ring, when worn constantly, generally requires a durable stone.
Color is a favorite approach to choosing a gem. One's color preference can supercede most other considerations, so a bit of education about the variety of color found in gemstones can help overall. Sapphires come in many colors, not just the well known blue. They are durable as well as they are just below diamonds on the hardness scale. Diamonds are still hundreds of times harder than sapphires. Colored diamonds are more easily available due to techniques used to manipulate the atomic structure of a diamond, this type of treatment is called HPHT or high pressure heat treatment. But the natural "fancy color" diamond occurs as shades of yellow, green, blue, brown, pink, orange, or red, the latter being the most rare. Any purchase of diamond should include a certfication as to its natural or treated state. Now certification is required to sell diamonds proving these were not acquired illicitly as a result of torture and killing of innocents.
Another consideration is whether the stone is natural or man-made. Natural stones are not "created" by a process, nor are they generally manipulated by irradiation (exposure to light) but still can be "treated" by heat to affect color. Heat treatment is common practice and has been in use for over three thousand years whereas irradiation is much more recent. Irradiation is commonly used to affect color change, followed by heat treatment to color the irradiated colorless topaz to blue. Most treatments are permanent but some are not, such as applying colored oil to a stone (done with rubies & emeralds) to enhance their appearance. Sometimes heat is applied to heal internal fractures, sometime a fill or flux is a part of this process. This flux can be detected by an simple eye loupe as it is opaque in appearance. Stones that are seriously manipulated in this manner are of far lesser value than untreated stones. Some design houses use only untreatead and natural stones. One is Cartier. An unheated Ceylon sapphire of moderate size ( 2-3 carats) is extremely valuable, appraising in the ballpark of $60,000.
Man made or lab created stones are very inexpensive. They can satisfy one's need for a colorful and durable stone but the buyer needs to be made aware of their origin as man made. I was recently in a store at a mall which advertised "pink sapphire" but which was never disclosed as lab created until I spoke with the corporate office on the store phone. As the stone was set in gold with small diamonds, it had a high price tag. It is never ethical for a retail store to leave out this information when presenting an item to the customer. Especially now as many lab created rubies for instance can bear the typical inclusions of a natural ruby such as silk (fine layering of rutile). Even quartz is today is man made: purple amethyst made in Japan is a lovely dark color. But just as lovely and far more intense purple is natural Namibian amethyst, this including red flashes at the beveled edges.
If you have more questions for a gemologist, write your request to gembroker1@yahoo.com and I will do my best to reply as a part of my newsletter at gembroker.net
Ms Munich is Gemmologist candidate for Fellow of the British Gemmological Association in London.