Star Sapphire is a variety of Corundum that displays an optical phenomenon known as asterism, where a six-rayed star appears on the surface when viewed under a direct light source. This effect is caused by tiny, needle-like inclusions of rutile aligned in multiple directions within the crystal lattice. Collectors should look for a centered, sharp star that moves smoothly across the cabochon-cut dome.
Is this star sapphire?
4-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch star sapphire with a known reference. Star Sapphire sits at Mohs 9, softer than the next harder reference, harder than the previous one.
- 2Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 3Match the color rangeCompare against the expected color range: blue, gray, violet, black, white.
- 4Look at form & habitCrystal system: trigonal. Typical habit: hexagonal prisms.
Often confused with
Star Sapphire vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside star sapphire
Minerals reported to co-occur with star sapphire. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Al₂O₃
- Mohs hardness
- 9
- Density
- 3.98-4.1 g/cm³
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Trigonal
- Crystal habit
- Hexagonal prisms
- Cleavage
- None
- Rarity
- Rare
- Uses
- Gemstone, Collector
- Host rock
- metamorphic rocks
Where rockhounds find star sapphire
Classic worldwide localities
- Sri Lanka
- Myanmar
- Madagascar
- Thailand
- Tanzania
Field-hunting tip
Where it forms: metamorphic rocks. If you start seeing zircon, rutile, spinel in float, you are in the right ground. Typical habit: hexagonal prisms. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify star sapphire?+
Where is star sapphire found?+
What rocks look like star sapphire?+
What minerals are found with star sapphire?+
What kind of rock does star sapphire form in?+
What is star sapphire used for?+
Find star sapphire on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
