Opal jewelry has long been admired for its incredible display
of color. As October's birthstone, it is often worn as round, oval or freeform
cabochons set in a variety of mountings. Opal jewelry is often classified as
having precious, fire or common gemstones. There are many other
classifications of opal based on their varying characteristics, such as black
opal, Mexican water opal, honey opal and moss opal. The value of an opal lies
in the size of the stone, the colors it displays and the pattern of the
colors. 
The difficulty in finding specimens with all three attributes is that
opals occur as very thin seems in rock or in white opals without fire. These
seems can be utilized by cementing the thin layer to a backing such as
obsidian or ceramic. This variety is known as a doublet. Using a black cement
will make the colors appear stronger. If a quartz cabochon is then cemented on
top of the opal, the stone is called a triplet. These varieties are frequently
used in opal jewelry, especially rings.
One of the most curious things about opals is that they
contain varying amounts of water. The amount can be between 5% and 30%
depending on the specimen. Because of this, opal jewelry should be kept in a
humid environment. Additionally, opals do not have any specific crystal
structure. Opal does possess a structure, however, it consists of random
chains of silicon and oxygen packed into extremely small spheres. In superior
specimens these spheres are organized into pockets of equal size and regular
concentration. This organized grouping creates the various colors frequently
sought after in opal jewelry.
The Moh's scale of hardness classifies opal as a 5.5-6.5. Much
care should be taken with opal jewelry. Because opals are hydrous, they must
be kept hydrated or else they may splinter or crack. Also, opals are very
heat-sensitive. When having your opal jewelry repaired, make sure that care is
taken to prevent dehydrating your gems. Do not use gem cleaner to clean your
opal jewelry. Use water and a soft polishing cloth to maintain its brilliance.
Applying mineral oil to your opal may also help maintain its hydration and
enhance its color temporarily.
