Rankinite is a rare calcium silicate mineral typically formed during the high-temperature metamorphism of limestone near igneous intrusions. It is usually found as massive or granular aggregates and is highly prized by collectors of contact metamorphic minerals for its scarcity.
Is this rankinite?
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 rankinite with a known reference. Rankinite sits at Mohs 5.5 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Rankinite leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Rankinite typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: colorless, white.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: granular, massive, or as rare prismatic crystals.
Often confused with
Rankinite vs. its common look-alikes — and how to tell them apart in the field.
Often found alongside rankinite
Minerals reported to co-occur with rankinite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Ca₃Si₂O₇
- Mohs hardness
- 5.5
- Density
- 2.95 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Transparent
- Crystal system
- Monoclinic
- Crystal habit
- Granular, Massive, Or as Rare Prismatic Crystals
- Cleavage
- Distinct in One Direction
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Contact Metamorphic Zones in Limestone
- Typical price
- $50-300 per specimen depending on size and quality
Where rockhounds find rankinite
Classic worldwide localities
- Carlingford, Ireland
- Crestmore, California, USA
- Kilchoan, Scotland
- Fuka, Japan
Field-hunting tip
Look in contact metamorphic zones in limestone country — that is the host setting where rankinite typically forms. If you start seeing larnite, gehlenite, wollastonite in float, you are in the right ground. Field specimens usually show a granular, massive, or as rare prismatic crystals habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find rankinite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
