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, so Rilandite leaves yellowish-brown, Jasper leaves white; luster reads resinous on Rilandite and waxy on Jasper.
How to tell apart: Streak differs, so 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, 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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