Calcite onyx is a banded, massive variety of calcite formed by the precipitation of calcium carbonate from cave waters or hot springs. It is easily distinguished from true silicate onyx by its softness, as it can be scratched with a knife and reacts readily with dilute hydrochloric acid.
Is this calcite onyx?
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 calcite onyx with a known reference. Calcite Onyx sits at Mohs 3 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Calcite Onyx leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Calcite Onyx typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: white, brown, tan, yellow, green.
- 5Look at form & habitCrystal system: trigonal. Typical habit: banded massive.
Often confused with
Calcite Onyx vs. its common look-alikes — and how to tell them apart in the field.
Often found alongside calcite onyx
Minerals reported to co-occur with calcite onyx. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- CaCO₃
- Mohs hardness
- 3
- Density
- 2.71 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Trigonal
- Crystal habit
- Banded Massive
- Cleavage
- Perfect Rhombohedral
- Fluorescence
- Often Fluorescent White, Yellow, Or Pink Under UV
- Rarity
- Common
- Uses
- Lapidary, Decorative, Collector
- Host rock
- Limestone Caverns and Hot Springs
- Typical price
- $5-50 for small polished pieces or spheres
Where rockhounds find calcite onyx
1 mapped spotsClassic worldwide localities
- Mexico
- Pakistan
- USA
- Italy
- Turkey
Field-hunting tip
Look in limestone caverns and hot springs country — that is the host setting where calcite onyx typically forms. If you start seeing aragonite, dolomite, quartz in float, you are in the right ground. Field specimens usually show a banded massive habit, so train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Tennessee — start trip planning there.
Common questions
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Find calcite onyx on the map
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