Precious vs. common

Precious opal shows play‑of‑color; common opal (a.k.a. potch) does not—its silica spheres are disordered or too small. Both are hydrated silica and can share the same host rock.

Named varieties

• Black opal — dark body tone enhances color contrast (classic at Lightning Ridge).
• White/opalescent opal — pale body with pastel flashes (Coober Pedy classic).
• Crystal opal — transparent to translucent body with vivid internal color.
• Fire opal — orange to cherry body color (Mexico); may or may not show play‑of‑color.
• Boulder opal — precious seams in ironstone matrix (Queensland).
• Matrix opal — color disseminated through porous host; sometimes darkened by tradition (e.g., Andamooka matrix).
• Hydrophane opal — porous, water‑absorbing (notably Ethiopian). Transparency and color can shift when wet.
• Opalized fossils — shells, wood, even invertebrates replaced or infilled by opal.

Silica on the move

Rainwater leaches silica from rocks and carries it as an ultra‑fine solution. In voids—cracks, cavities, fossils—the silica slowly deposits as a gel with water trapped in its network.

Sphere assembly

As the gel matures, silica forms sub‑micron spheres. Where conditions are steady, spheres settle into ordered arrays; where conditions fluctuate, the arrangement becomes disordered.

From gel to gem

Slow dehydration and compaction produce solid opal. Ordered arrays diffract light → precious opal; disordered areas are potch. Many stones show both.

Diffraction & interference

Regularly packed silica spheres (and the gaps between) act like a 3D diffraction grating. Different wavelengths reflect at different angles, so when you tilt the stone the spectrum rolls across the surface.

Sphere size = color range

Approximate rule: larger spheres → red/orange, smaller spheres → blue/violet. Mixed sizes create a richer palette. Disorder or too‑tiny spheres kill the color.

Body tones

• Black to dark gray — maximal contrast; colors look electric.
• White/cream — softer pastels; classic “milky” opal.
• Orange/red — fire opal body color (with or without play‑of‑color).
• Crystal — near‑colorless body; internal color floats in space.

Pattern words

• Pinfire — tight, starry pinpoints.
• Broad flash — large areas light up at once.
• Rolling flash — color band rolls as the stone tilts.
• Harlequin — mosaic of angular patches (rare).
• Mackerel sky / flagstone / Chinese writing — descriptive nicknames for distinctive patch shapes.

Photo tip: Use one small light source and a dark backdrop; move the stone, not the light. Opal loves choreography.

Micro‑textures

At 10×, precious areas show crisp, angular or granular color patches that shift with tilt; potch looks uniform and non‑sparkly. Internal “clouds” are common—minute sphere arrays and tiny bubbles.

Hydrophane clues

Porous opal (notably Ethiopian) may show spiderwebby translucency or patchy transparency variations. A dampened surface can turn glass‑clear until it dries back.

Strain & crazing

Fine, random micro‑cracks (crazing) can appear in unstable material. View with side‑light; if present, plan gentle display rather than daily wear.

Synthetic opal

Lab‑grown with the same silica‑sphere structure (often polymer‑stabilized). Under magnification, many show regular “lizard‑skin” or “chicken‑wire” patterning or columnar patches, and colors that repeat with uncanny uniformity.

Simulants

Opalite glass, Slocum Stone, and various plastics produce opal‑like color but lack true diffraction micro‑structure. Bubbles, mold lines, and overly even color are clues.

Doublets & triplets

Thin precious opal is often laminated: a doublet = opal + dark backing; a triplet adds a clear cap (quartz/glass). Side view shows the layers; avoid soaking—adhesives can fail.

Treated matrix

Some porous matrix opals (e.g., Andamooka) are carbon‑treated (“sugar‑acid” or smoke) to darken the background and boost contrast. Properly disclosed treatment is part of their story.

Quick checklist

• Natural precious opal: non‑repeating patterns, organic patch shapes.
• Synthetic: highly regular “tile” or “lizard‑skin” motifs.
• Doublet/triplet: visible layers; domed clear cap in triplets.

At‑home observations

Loupe the side for laminations; check back and edge for uniform dye lines; look for pattern repetition that suggests lab growth.

Australia

Lightning Ridge (black opal), Coober Pedy (white & crystal), Andamooka (matrix), Queensland (boulder opal in ironstone). The Great Artesian Basin’s sedimentary rocks are a world‑class opal playground—including opalized fossils.

Elsewhere

Ethiopia (Welo hydrophane; Shewa), Mexico (Querétaro fire opal), Brazil, USA (Nevada’s Virgin Valley black opal; Idaho), Honduras (black matrix), and historic localities like Dubník, Slovakia.

Everyday care

• Protect from hard knocks; opal is brittle.
• Avoid rapid temperature swings and very dry/heated storage.
• Rings and bracelets: choose protective or bezel settings; remove for rough tasks.

Cleaning

• Use lukewarm water + mild soap + a soft cloth. Rinse and dry.
• No ultrasonic or steam cleaners. Avoid harsh chemicals and bleach.
• For doublets/triplets, keep away from prolonged soaking—water can creep into laminations.

Hydrophane specifics

• Porous stones may absorb water, oils, and dyes—avoid lotions and prolonged immersion.
• Appearance can change temporarily when wet; allow to air‑dry naturally.
• Repeated wet/dry cycling isn’t ideal—aim for a stable environment.