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. Thorianite leaves a grayish-black streak.
- 3Read the lusterHold the specimen under a strong light. Thorianite typically shows a submetallic luster.
- 4Match the color rangeCompare against the expected color range: black, brownish-black, reddish-brown.
- 5Look at form & habitCrystal system: cubic. Typical habit: cubes, octahedrons, water-worn pebbles.
Often confused with
Thorianite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Streak differs — Thorianite leaves grayish-black, Uraninite leaves brownish-black to greenish-black; luster reads submetallic on Thorianite and submetallic to dull on Uraninite.
How to tell apart: Streak differs — Thorianite leaves grayish-black, Zircon leaves white; luster reads submetallic on Thorianite and adamantine 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
- 9.0-10.0 g/cm³
- Streak
- Grayish-black
- Luster
- Submetallic
- Transparency
- Opaque
- Crystal system
- Cubic
- Crystal habit
- Cubes, Octahedrons, Water-worn Pebbles
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector, Thorium Source
- Host rock
- Pegmatites, Contact Metamorphic Rocks, Alluvial Deposits
- Typical price
- $50-500 thumbnail depending on size and crystal quality
Where rockhounds find thorianite
Classic worldwide localities
- Sri Lanka
- Madagascar
- Russia
- Canada
Field-hunting tip
Look in pegmatites, contact metamorphic rocks, alluvial deposits country — that is the host setting where thorianite typically forms. If you start seeing zircon, ilmenite, spinel in float, you are in the right ground. Field specimens usually show a cubes, octahedrons, water-worn pebbles habit, so 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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