Birthstones - October  (Opal or Tourmaline)

Opal

Chemical Composition: SiO2nH2O - Hydrous Silicon Dioxide
Mohs Hardness: 4.5-6.5
Specific Gravity: 2.15
Refractive Index: 1.45

Opal is a birthstone for the month of October and the gemstone representing the 14th wedding anniversary. Other birthstones that may be used for the month of October are pink tourmaline or rose zircon.
 
Precious, or gem opal, which displays "opalescence" (spectral color, iridescence that changes with the angle at which the gem is viewed), is the kind of opal with which most people are familiar. There are three major kinds:

1. White Opal - White opal is an opaque stone in which the colors appear as flashes or speckles.
2. Black Opal - Black opal contains fire with a dark body color. These are less common and tend to be costly.
3. Opalized Organic Material - Opalized organic material may consist of wood, plant stem, bone or shell that has been petrified under extremely rare conditions.

There are several other varieties of opal available today. Among them are:

Crystal Opal
Crystal opal, the next most costly type of opal, is transparent with flashes of color. It is highly valued for the brilliance of its colors and the fact that many layers of color can be seen within the stone.

Boulder Opal
Another unusual type of opal is boulder opal, which has opal with an ironstone host rock matrix which creates a natural dark background to view its fire. These sometimes occur in "splits," a matched pair of opals created when a piece of boulder opal is split along the opal vein. These are particularly favored for earrings, since they are mirror images of each other.

Fire Opal
Fire opal is transparent or translucent with an orange or red body color. Fire opals are named for their reddish color but are opalescent. Be careful not to misinterpret the term "fire" as iridescence, because in precious opal (with a play of iridescent color) the play itself is also called fire. Much of the world's fire opal is mined in Mexico.


Care & Cleaning of Opals

Because opal is a relatively soft stone, it should be treated with special care. Opal is both brittle and heat-sensitive and, therefore, should not be immersed in hot water or in an ultrasonic cleaner. While no gemstone is indestructible, opal is a bit softer and more fragile than many gemstones. However, with proper setting and ongoing care, an opal can last a lifetime. Generally, the thicker the opal, the better. Look for settings that protect the opal, such as bezel settings where the metal holds the stone all the way around its edges. Avoid storing opals in a bank safe deposit box. The atmosphere in bank vaults is purposely kept dry in order to protect important paper documents. Unfortunately, this climate hastens the drying (and destruction) of precious opal. We suggest that you bring your jewelry to Gillespie Jewelers at least three times each year for a complimentary inspection and cleaning.