Rumoiite is a rare gold tin mineral that forms silver white irregular grains. It occurs in fluvial placer deposits. Associated minerals include gold, shosanbetsuite and yuanjiangite. It was first described from the Shosanbetsu river, Rumoi province, Hokkaido, Japan.
Is this rumoiite?
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 rumoiite with a known reference. Rumoiite sits at Mohs 2.5, softer than the next harder reference, harder than the previous one.
- 2Read the lusterHold the specimen under a strong light. Luster: metallic.
- 3Match the color rangeCompare against the expected color range: silver white, white.
- 4Look at form & habitCrystal system: orthorhombic. Typical habit: irregular grains.
Often confused with
Rumoiite vs. its common look-alikes, and how to tell them apart in the field.
All properties
- Chemical formula
- AuSn₂
- Mohs hardness
- 2.5
- Density
- 10.1 g/cm³
- Colors
- Luster
- Metallic
- Transparency
- Opaque
- Crystal system
- Orthorhombic
- Crystal habit
- Irregular grains
- Rarity
- Rare
- Uses
- Collector, Scientific research
- Host rock
- hydrothermal veins in ultrabasic rocks
Where rockhounds find rumoiite
Classic worldwide localities
- Rumoi, Hokkaido, Japan
Field-hunting tip
Where it forms: hydrothermal veins in ultrabasic rocks. If you start seeing distribution from minamichiyoda, shosanbetsu village, rumoi subprefecture in float, you are in the right ground. Typical habit: irregular grains. Train your eye for that shape before scanning the outcrop.
Common questions
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Find rumoiite on the map
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