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October (Opal)
- White, gray, blue, green, orange
- Mohs scale of hardness 5.5 - 6.5
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. They actually 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.
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