Monthly Mineral Spotlight – October

Opal: Fire, Color, and Hidden Worlds

Opal is one of the most enchanting materials a rockhound can discover. Known for its rainbow flashes, pastel hues, and incredible variety, opal has inspired myths, art, and gemstone collecting for centuries. October is the perfect time to explore this brilliant mineraloid.

What Is Opal?

Opal is a hydrated silica mineraloid made of microscopic silica spheres. When these spheres are uniform and aligned, they diffract light to produce play-of-color — the hallmark of precious opal.

Irregular spheres create common opal, which displays solid color without fire.

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Quick Identification Guide

• Precious opal shows play-of-color
• Common opal has solid color
• Hardness 5.5–6.5
• Conchoidal fracture
• Translucent edges in many varieties
• Hydrophane opal absorbs water

Varieties of Opal

Precious Opal

Brilliant flashes of spectral color.

Fire Opal

Orange to red; may or may not contain play-of-color.

Moss Opal

Common opal with leafy or mossy inclusions.

Opalized Wood

Fossil wood replaced with opal (common in Nevada).

Hyalite Opal

Clear, glassy, often fluorescent.

Boulder Opal

Precious opal veins in ironstone.

Matrix Opal

Opal dispersed through porous host rock.

How Opal Forms

Opal forms when silica-rich water fills cracks, voids, or cavities in rock. As the water evaporates, silica spheres remain. The size and arrangement of these spheres determine whether the opal is precious or common.

Opal forms in:

• volcanic ash beds
• sedimentary basins
• basalt cracks
• fossil cavities
• opalized wood deposits

Major Opal-Producing Areas

United States

• Northern Nevada — famous for opalized wood and black precious opal
• Oregon — Opal Butte and volcanic deposits
• Idaho — common opal in many colors

Australia (global leader)

• Coober Pedy — white and crystal opal
• Lightning Ridge — black opal
• Queensland — boulder opal fields
• Andamooka — matrix opal

Mexico

Known for transparent fire opal.

Ethiopia

Hydrophane opal that absorbs water.

Rockhounding Tips for Collecting Opal

• Bring water to rinse material
• Use care — opal can crack if exposed to heat
• Hydrophane opal changes appearance when wet
• Look for opalized wood fragments in volcanic regions
• Some opal fields are fee-dig areas

Lapidary Notes

• Opal is soft—polish gently
• Use light pressure when cutting
• Avoid overheating; it may craze
• Stabilization is sometimes used for porous material

FAQs About Opal

Why does opal show color?

Play-of-color comes from light diffracting off silica spheres.

Is hydrophane opal good for jewelry?

It can be—but it absorbs water, so care is needed.

Is Nevada opal really petrified wood?

Yes — much of Nevada’s precious opal began as buried wood replaced by silica-rich fluids.

Fun Fact

Coober Pedy is so hot that most residents live in underground homes called “dugouts,” carved directly into old opal workings.

Conclusion

Opal’s diversity, beauty, and geological complexity make it one of the world’s most treasured mineraloids. From Nevada’s opalized wood to the rainbow flashes of Australia’s black opal, October is the perfect month to explore this magical material.