What’s in the Box? Opal

October’s birthstone can be a bit confusing: what is “precious” opal, anyway? Is it different from fire opal? And what kinds of opals are “common”? This post is a short primer for all your opal terminology needs.

Precious Opal in Thunderegg

This close-up of a thunderegg filled with opal from Opal Butte, Oregon, shows the flecks of blue, purple, and green called “play of color.” This play of color makes part of this stone precious opal. But not all opals have this feature!

Unlike true minerals, opal does not have a rigidly repeating, set internal structure. Instead, it’s made up of many microscopic beads of silica that get jammed together and bound up with water. When these microscopic beads stack up in a nice, orderly fashion, they create the beautiful flashing hues you see above. Nature is not always so organized, however–when the beads are stacked randomly, or when they have very different sizes, the opal becomes milky and clouded.

Common opal is the term for white stones without any flashing color. Hyalite is perfectly clear. (Its structure is so disorganized the its silica isn’t even found in beads–instead, it is amorphous like a jelly.) Fire opal is transparent but is red or orange in color.

These four types of opal–precious, common, fire, and hyalite–are present in the photo above. Can you spot them all?


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