Rilandite is a rare chromium-bearing silicate mineral typically found as thin crusts or films on fossil wood in sandstone. It is distinguished by its distinct resinous luster and deep reddish-brown color, most commonly identified in the Morrison Formation of Colorado.
Is this rilandite?
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 rilandite with a known reference. Rilandite sits at Mohs 3-4 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Rilandite leaves a yellowish-brown streak.
- 3Read the lusterHold the specimen under a strong light. Rilandite typically shows a resinous luster.
- 4Match the color rangeCompare against the expected color range: dark red, reddish-brown.
- 5Look at form & habitCrystal system: amorphous. Typical habit: encrusting.
Often confused with
Rilandite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Jasper is the harder of the two (Mohs 6.5-7 vs. 3-4); streak differs — Rilandite leaves yellowish-brown, Jasper leaves white; luster reads resinous on Rilandite and waxy on Jasper.
How to tell apart: Streak differs — Rilandite leaves yellowish-brown, Cinnabar leaves scarlet; luster reads resinous on Rilandite and adamantine on Cinnabar.
Often found alongside rilandite
Minerals reported to co-occur with rilandite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Cr₆Si₂O₁₁·5H₂O
- Mohs hardness
- 3-4
- Density
- 2.9-3.1 g/cm³
- Colors
- Streak
- Yellowish-brown
- Luster
- Resinous
- Transparency
- Opaque
- Crystal system
- Amorphous
- Crystal habit
- Encrusting
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Sedimentary Sandstone
- Typical price
- $50-300 per specimen
Where rockhounds find rilandite
Classic worldwide localities
- Colorado, USA
Field-hunting tip
Look in sedimentary sandstone country — that is the host setting where rilandite typically forms. If you start seeing quartz, calcite in float, you are in the right ground. Field specimens usually show a encrusting habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find rilandite on the map
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