Semiblack Opal Identification Guide

What Semiblack Opal Looks Like

Semiblack opal (also called "semi-black" or grey-based opal) is precious opal with a dark body tone that falls between black opal and light opal — on the standard opal body-tone scale it sits in the N5–N6 (dark grey) range, darker than light opal but lighter than true black (N1–N4). The dark background makes the play-of-color (flashes of spectral color) stand out vividly. Luster is vitreous to resinous, transparency ranges from opaque to slightly translucent, and the stone shows shifting reds, greens, blues, and oranges as it is tilted.

Step-by-Step Field ID Checklist

1. Judge the body tone — the background (ignoring the color flashes) should read as dark grey, not jet black and not milky white.
2. Tilt for play-of-color — genuine precious opal flashes multiple spectral colors that move and change with angle.
3. Check the back and sides — a single solid piece (not a layered sandwich) indicates a solid opal, not a doublet/triplet.
4. Inspect luster — glassy to slightly waxy.
5. Look for a straight glue line — a flat, dark joined backing signals a doublet.
6. Hardness — opal is Mohs ~5.5–6.5, softer than quartz.

Key Diagnostic Tests

• Body-tone comparison: use an opal body-tone chart; semiblack = N5–N6 grey.
• Mohs hardness: ~5.5–6.5 — will be scratched by quartz; do not scratch a finished gem.
• Play-of-color: true diffraction colors shift with angle; printed/painted fakes do not.
• Profile inspection: view the stone edge-on under magnification to spot a flat join (doublet/triplet) versus a solid dome.
• Density: low (~2.0–2.2 g/cm³); feels light.
• Backing test: a black painted or ironstone backing on a thin opal slice indicates an assembled stone, not natural semiblack.

Common Look-Alikes and How to Tell Them Apart

• True black opal: darker body tone (N1–N4); semiblack is distinctly grey, not black. Compare against a body-tone chart.
• Doublet/triplet: a thin opal layer glued to a dark backing mimics a dark body tone — look for a straight glue line and flat base in profile; semiblack solid opal is uniform through its thickness.
• Boulder opal: opal naturally attached to ironstone host; the dark tone comes from the natural rock backing, not from the opal's own body color.
• Crystal/light opal: transparent or light-bodied with play-of-color on a pale base; lacks the dark grey background.
• Imitation opal (Gilson/resin): shows an unnaturally regular 'snakeskin' color pattern or columnar structure under magnification.

Where Semiblack Opal Is Typically Found

Semiblack opal comes chiefly from Australian opal fields, especially Lightning Ridge (NSW) and nearby black-opal areas, plus Mintabie and some other fields that produce dark-bodied stones. Material from these fields spans the body-tone scale, with semiblack representing the grey middle range between prized black opal and common light opal.