Hardness in gemology refers to the Mohs Scale of Hardness, which is a relative scale as opposed to a proportional scale. For example, Diamonds have one of the highest Hardness ratings at 10, but they are not 2 times as hard as Apatite, which has a hardness rating of 5. As a means of comparison, regular steel has a hardness rating of 4, while hardened steel is 8.
In practical terms from a buyer’s standpoint, use hardness as a measure of “scratchability” — something to be considered when buying gemstones. For a wedding band or engagement ring that’s worn everyday, you probably want to consider one of the harder stones. This is one reason diamonds are so popular in wedding jewelry, but other stones work as well in more unique engagement rings created using sapphires (9), emeralds (7.5), and topaz (8) for example.
In geology, “inclusion” refers to any material — gas, liquid, another mineral, even an insect or seed — trapped inside a geologic formation. In gemology, inclusions affect the clarity, character, and value of gemstones. But ask your gemologist about the particular gemstone you are considering, as the way in which inclusions affect a stone’s value varies widely. In some cases, such as the star sapphire, the inclusion makes the stone more valuable. Colored gemstones are classified in three levels with regard to inclusions:
- Type I colored stones include stones with very little or no inclusions. They include aquamarines, topaz and zircon.
- Type II colored stones include stones that often have a few inclusions. They include sapphire, ruby, garnet and spinel.
- Type III colored stones include stones that almost always have inclusions. Stones in this category include emerald and tourmaline.
The Refractive Index is a measure of how quickly light travels through a substance. The higher the Refractive Index value, the faster light passes through and the greater the potential for brilliance and luster in the gem, depending on how it is faceted or cut.
Specific gravity refers to a measure that compares the weight of the gem in air with the weight of the gem suspended in water. Gemologists use this measure as one means of ascertaining whether a stone is genuine.