Gemstones cannot typically be classified simply by their color, as the same gem may occur in many shades. To properly identify a gem it is necessary to look to other characteristics of the material. A brief list of the most commonly used physical characteristics appears below. To view information on a particular stone either scroll down through the list below, or click on one of the links to the left. Additional information on the gem properties is included at the end of the list, as is a list of links to other sites.
Garnet is actually a group of minerals with closely related chemical and physical properties. The "X" element is typically Mg, Fe, or Ca. The "Y" element is typically Al, but may also be Fe3+ or Cr. A bright red variation, Pyrope, has the chemical formula Mg3Al2Si3O12.
Sapphire is one of the two varieties of Corundum, the other variety being Ruby. Sapphire includes all colors of Corundum except for red stones, which are referred to as Ruby. Sapphire is the most highly valued of the blue gemstones.
Topaz is an aluminum silicate material which occurs naturally in a number of colors. Natural stones may also be heat treated to yield pink, blue, and purple stones. Citrine [Quartz] may easily be confused with Topaz, although the latter is more valuable. The "X" component may be F or OH.
Tourmaline occurs across the entire spectrum of gemstone colors, and may even be multicolored. As with Garnet, Tourmaline is actually a group of closely related group of minerals. The "X" component is Na or Ca. The "Y" component may be Mg, Li, Al, or Fe2+.
SPECIFIC GRAVITY The specific gravity of a substance is a measure of its density [weight per given volume] as compared to water. A substance with a specific gravity of 1.00 is exactly as dense as water. A higher specific gravity means that the substance is more dense than water, while a specific gravity of less than 1.00 means that the substance is less dense than water. Most gemstones have a specific gravity of between 3 and 4, thus are three to four times denser than water. The value of a gemstone is not typically determined by its specific gravity. This measure is useful primarily in distinguishing between different types of minerals with similar optical characteristics. HARDNESS If one substance will scratch another, the first substance is said to be the harder of the two. A subjective scale to rate the relative hardness of mineral specimens was developed by Frederick Mohs in the early 1800's. This scale, named for its creator, is still used today. The Mohs scale ranks minerals on a scale of one to ten, based on the ability of the higher-rated materials to scratch the lower-rated materials. The minerals used in the Mohs scare are:
REFRACTIVE INDEX [RI] The refractive index is a measure of the degree to which light is refracted [bent] as it through a particular stone. Light rays are deflected when they pass from on air [or other medium] into a gemstone, and the measure of this deflection can be used to identify the gem material. The refractive index is also used in determining how to shape a gemstone. If the RI is known, the gem cutter can adjust the angles at which facets are ground onto the stone to maximize the effect of reflected and refracted light within the stone.