Geologically speaking, precious opal is a rare find. With so few localities producing gem-quality opal, scientists have had some trouble understanding exactly how this material develops.
Learn how opals form, why they show play of color, and some mysteries about their formation that scientists are still investigating.
Chemically speaking, opal is a form of hydrated silica, with the chemical formula SiO₂ · nH₂O. Unlike most gemstones, opal isn’t crystalline. That means the silica doesn’t form in a nice, neat crystal structure. Instead, it forms as microscopic, amorphous spheres. Think of opal micro-structure as more like balls of yarn than interlocking toy bricks.
In gem-quality opal, the play of color arises from silica spheres with a uniform size and closely packed structure. Common opal or “potch,” in contrast, has silica spheres with a wide range of sizes jumbled together.
Like any gemstone, opals need specific ingredients and geological conditions to form. In this case, there needs to be dissolved silica that comes out of solution and forms opal. First, water filters into the ground and dissolves quartz sand, becoming a silicic acid solution.
There are several theories about what happens next.
The most widely accepted theory involves seasonal changes to the groundwater table. According to this theory, groundwater near the surface evaporates seasonally, allowing a thin layer of opal to form in pores and seams in the rock. Over time, opal grows in these spaces in near-surface rocks, forming the seams and nodules (nobbies) that miners dig today.
The time opal formation takes is one big mystery. In Australia’s opal fields, scientists estimate opals began forming about 30 million years ago and calculate a growth rate of 5-6 million years for 1 cm of opal.
However, a study of biogenic matter from cracks within a sample of Lightning Ridge black opal yielded a radiocarbon age of 1,000 to 7,000 years, much younger than other estimates. Ultimately, the time it takes to form opals is still not well-understood.
Opal types are primarily based on the gem’s body color. This is the color that shows under a precious opal’s play of color, the flashing or moving colors across its surface. However, sometimes the form, matrix, or inclusions define the type. (Opals that show play of color are called “precious opals,” while those that don’t are called “common opals”).
The following types are the most frequently used for jewelry:
– White (light) Opal
– Gray (dark) Opal
– Black Opal
– Boulder Opal (Opal in matrix)
– Crystal Opal
– Fire Opal
Opals occur in a wide variety of body colors, and many of these have their own names. Although most often named after popular gemstones they resemble, some varieties are just named for their colors. While opals with attractive body colors can show play of color, these opal types are also popular without it. Although they’re less valuable, opals with blue, pink, or green body colors still make lovely jewelry stones.
From the International Gem Society