Overview
ABOUT OPAL
Because opal has the colors of other gems, the Romans thought it was the most precious and powerful of all. The Bedouins believed that opals contained lightning and fell from the sky during thunderstorms. When Australia’s mines began to produce opals commercially in the 1890s, it quickly became the world’s primary source for this October birthstone.
BIRTHSTONES & ANNIVERSARIES
0.2 Microns
Grids of silica spheres 0.2 microns in size create red play-of-color flashes.
20% Water
Opal contains up to 20% water trapped in its silica structure.
1829
The novel “Anne of Geierstein” gave opal a reputation of being unlucky.
FACTS
Mineral: Hydrated Silica
Chemistry: SiO2•nH2O
Color: All colors
Refractive index: 1.37-1.47
Birefringence: None
Specific gravity: 2.15 (+0.08, -0.90)
Mohs Hardness: 5 to 6.5
TREATMENTS
There are a number of processes used to alter the color, apparent clarity, or improve the durability of gems.
SYNTHETICS
Some gemstones have synthetic counterparts that have essentially the same chemical, physical, and optical properties, but are grown by man in a laboratory.
IMITATIONS
Any gem can be imitated—sometimes by manmade materials or by natural materials chosen by man to impersonate a particular gem.
WHY WE LOVE THIS GEMSTONE
1
Formation
When opal formed, silica gel filled crevices in rock. As water evaporates, the silica is deposited in the form of tiny spheres.
2
Interaction with Light
Opal’s flashing play-of-color is caused by diffraction of light by silica spheres stacked like tiny Ping-Pong balls in a box.
3
Unique
Opal’s arrays of silica spheres form a fantastic variety of patterns and colors. No two opals are exactly alike.
Quality Factors
Play-of-color, intensity, and pattern are important value factors.
COLOR
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CLARITY
CUT
CARET WEIGHT
OPAL QUALITY FACTORS: THE COMPREHENSIVE GUIDE
Research
Explore sources, gemological research, and the role of gems in history.
Recommended Reading
Opal & Gemstone Jewelry: Cutting, Designing, Setting: a Step-by-Step Lapidary Instructional Guide
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