Thorianite is a dense, highly radioactive mineral typically found as small cubic or octahedral crystals. It is most commonly sourced from alluvial gem gravels where it occurs as water-worn, heavy black pebbles. Collectors should store this mineral with caution due to its significant radioactive decay properties.
Is this thorianite?
5-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 thorianite with a known reference. Thorianite sits at Mohs 6.5-7, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: gray to greenish gray.
- 3Read the lusterHold the specimen under a strong light. Luster: submetallic when fresh, altering to resinous or hornlike.
- 4Match the color rangeCompare against the expected color range: brownish black, green, green black, gray green, yellow.
- 5Look at form & habitCrystal system: cubic. Typical habit: granular, pseudo hexagonal.
Often confused with
Thorianite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Thorianite leaves gray to greenish gray, Uraninite leaves brownish black, gray, olive-green, shining.
How to tell apart: Streak differs, so Thorianite leaves gray to greenish gray, Zircon leaves white; luster reads submetallic when fresh, altering to resinous or hornlike on Thorianite and vitreous to adamantine; greasy when metamict on Zircon.
Often found alongside thorianite
Minerals reported to co-occur with thorianite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- ThO₂
- Mohs hardness
- 6.5-7
- Density
- 9.7-9.8 g/cm³
- Colors
- Streak
- Gray to greenish gray
- Luster
- Submetallic when fresh, altering to resinous or hornlike
- Transparency
- Opaque, transparent in very thin fragments
- Crystal system
- Cubic
- Crystal habit
- Granular, pseudo hexagonal
- Cleavage
- {001}, poor
- Rarity
- Rare
- Uses
- Collector, Thorium source
- Host rock
- An accessory mineral in granite pegmatites, more rarely in carbonatites or serpentinites; commonly detrital in heavy mineral concentrates
Where rockhounds find thorianite
Classic worldwide localities
- Easton, Northampton Co., Pennsylvania, USA
- Nixon Fork and Wiseman districts, Alaska, USA
- Balangoda and near Kondrugala, Sri Lanka
- Galle district, Sri Lanka
- Bambarabotuwa, Sri Lanka
- Taolan̈aro (Fort Dauphin), Madagascar
Field-hunting tip
Where it forms: An accessory mineral in granite pegmatites, more rarely in carbonatites or serpentinites; commonly detrital in heavy mineral concentrates. If you start seeing thorite, zircon, ilmenite in float, you are in the right ground. Typical habit: granular, pseudo hexagonal. Train your eye for that shape before scanning the outcrop.
Common questions
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Find thorianite on the map
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