Star Opal Identification Guide
Identify star opal by its play-of-color arranged in a rare star pattern, low hardness and density, conchoidal fracture, and opal look-alikes.
Read the full Star Opal encyclopedia entry →
What Star Opal Looks Like
Star opal is a very rare form of precious opal in which the play-of-color is organized into a radiating, star-like pattern — a central point from which spectral colors fan out in rays. It should not be confused with star (asteriated) gems like star sapphire; in opal the 'star' is a distribution of play-of-color, not a reflected light star from needle inclusions. The body can be light (white opal) or dark (black opal base), with flashes of green, blue, red, and orange forming the rayed figure.
- Color: white, gray, or black body with multicolor play-of-color in a star arrangement
- Luster: vitreous to waxy/resinous
- Transparency: translucent to opaque
- Habit: amorphous hydrated silica; cut as cabochons
Step-by-Step Field-ID Checklist
- Confirm play-of-color first — tilt under light; true opal flashes spectral colors that move and change, not a fixed body tint.
- Look for the radiating pattern — colors arranged in a spoked or star-like figure around a center.
- Note that the 'star' shifts and flashes (it is play-of-color), unlike the steady reflected star of star sapphire.
- Heft it — opal feels light for its size.
- Inspect the edge for triplet/doublet construction (a flat dark backing or clear cap), common in thin opal.
Key Diagnostic Tests
- Mohs hardness: 5.5–6.5 — softer than quartz; does not scratch glass easily.
- Streak: white.
- Fracture: conchoidal; no cleavage (amorphous).
- Density: low, ~1.9–2.2 g/cm³ — notably light.
- Hydrophane behavior: some opal absorbs water and changes clarity; test cautiously.
- No acid reaction; not magnetic.
Common Look-Alikes and How to Tell Them Apart
- Star sapphire / true asteriated gems: show a steady reflected six-rayed star that glides with the light and have NO spectral play-of-color; they are far harder (9) and denser. Star opal's 'star' is moving rainbow play-of-color.
- Synthetic opal: shows a regular columnar/'lizard-skin' play-of-color pattern under magnification; natural star opal is irregular.
- Opalite / opal-glass imitation: man-made glass glows milky blue-to-orange with no true spectral play-of-color.
- Foiled or printed 'star' glass: a fixed, etched star with no color change; opal's pattern shifts as you tilt it.
- Common opal (potch): same mineral but no play-of-color and thus no star.
Where It Is Found
Star opal is exceptionally rare and is reported sporadically from major precious-opal fields, principally Australia (Lightning Ridge for black opal, Coober Pedy for light opal) and Ethiopia (Welo), wherever the silica spheres happen to align into a radiating play-of-color figure.
Frequently asked questions
How can you tell if it's real star opal?
Real star opal is precious opal: low hardness (5.5–6.5), low density (it feels light), conchoidal fracture, and shifting spectral play-of-color — here arranged in a radiating star figure that flashes and changes as you tilt it, unlike the steady reflected star of star sapphire.
What is the difference between star opal and star sapphire?
Star sapphire shows a fixed silvery six-rayed star from needle inclusions and has no rainbow play-of-color; star opal's 'star' is made of moving multicolor play-of-color, and opal is far softer and lighter.
Is star opal rare?
Yes, very. It requires the play-of-color to organize into a radiating star pattern, which is an unusual natural arrangement found only occasionally in major opal fields.
Star opal vs synthetic opal — how do I tell them apart?
Synthetic opal shows an extremely regular columnar or lizard-skin play-of-color under magnification; natural star opal has irregular, random color play even when it forms a star figure.
What does star opal look like?
A white, gray, or black opal whose green, blue, red, and orange play-of-color radiates outward from a center in a star-like, spoked pattern.