Common Opal Identification Guide

What Common Opal Looks Like

Common opal is hydrated silica (SiO₂·nH₂O) that lacks the rainbow play-of-color of precious opal. It comes in nearly every color: milky white, gray, tan, yellow, pink, green, blue, brown, and black. Luster is glassy (vitreous) to waxy, resinous, or dull. Transparency runs from translucent to opaque, sometimes with a soft internal glow (opalescence or a milky sheen) but never the flashing spectral fire of precious opal. It is amorphous, so it forms no crystals; instead it appears as nodules, seams, vein fillings, botryoidal crusts, or replacement of wood and fossils. Surfaces often show a smooth, gel-like, glassy texture.

Step-by-Step Field ID Checklist

1. Check for play-of-color. Tilt under light. If there is NO flashing spectral color (only a steady body color or milky glow), it is common opal, not precious opal.
2. Look at luster and feel. A glassy-to-waxy, smooth, almost greasy surface is typical.
3. Inspect the form. Massive, nodular, botryoidal, or vein-filling habit with no crystal faces supports opal.
4. Test hardness. Opal is softer than quartz (see tests).
5. Look at the break. Smooth conchoidal fracture with sharp edges.
6. Note any host. Often sits in seams or cavities within volcanic rock, sedimentary host, or as petrified wood.

Key Diagnostic Tests

• Hardness: 5.5–6.5, distinctly softer than quartz (7). A steel knife or quartz point may scratch it.
• Streak: White.
• Fracture: Conchoidal to uneven; no cleavage (amorphous).
• Magnetism: None.
• Acid: No reaction to dilute HCl (distinguishes from carbonates).
• Density: Low, ~1.9–2.3 g/cm³, noticeably lighter than quartz or chalcedony; feels light for its size.
• Water sensitivity: Hydrophane varieties may stick to the tongue and change clarity when wetted.

Common Look-Alikes and How to Tell Them Apart

• Precious opal: Same material but shows play-of-color; common opal does not.
• Chalcedony/agate: Harder (7), denser, often banded; opal is softer and lighter, and chalcedony lacks opal's gel-like glow.
• Glass/slag: Glass is harder, often shows bubbles and mold marks; opal is softer with conchoidal but earthier surfaces.
• Opalite (man-made): Manufactured glass with a blue Tyndall sheen and orange backlight; uniform and bubble-bearing.
• Calcite/howlite: Calcite fizzes in acid and is softer (3); howlite is chalky white with gray veining and very soft.

Where It Is Typically Found

Common opal forms from silica-rich groundwater in volcanic and sedimentary rocks worldwide. Notable sources include Peru (pink and blue opal), Mexico, the western United States (Nevada, Oregon, Idaho), Australia, Indonesia, and Madagascar, occurring as seams, nodules, geode linings, and petrified wood.