Calcite: properties, varieties and value factors

Key properties

Chemical formula	CaCO3
Crystal system	Trigonal
Mohs hardness	3
Toughness	Poor in single crystals; fair to good in aggregates
Refractive index	1.486-1.658
Birefringence	0.1720-0.1740
Optic character	Uniaxial negative; aggregates (marble) read as AGG with high spot birefringence (-)
Dispersion	0.0170
Specific gravity	2.65-2.75
Cleavage	Perfect rhombohedral in three directions - dominates breakage; often hidden in aggregates
Fracture	Conchoidal to uneven on massive material (cleavage rarely lets a clean fracture develop)
Habit	Rhombohedra, scalenohedra (dogtooth spar), nailhead crystals, massive marble, stalactitic, fibrous satin spar, onyx banded
Lustre	Vitreous to dull/earthy on aggregates; pearly on cleavages; polish: Vitreous to greasy/waxy
UV fluorescence	Highly variable - some material fluoresces red, orange, blue or yellow in SW/LW, often with phosphorescence; manganese impurities activate many examples
Pleochroism	None to weak (coloured varieties)
Cause of colour	Various impurities - iron and manganese tones, organic matter and clays in banded material; dyes in treated onyx marble
Typical inclusions	Doubled images through transparent crystals (extreme birefringence); fluid inclusions; cleavage streaks; banding impurities in marbles
Stability	Effervesces readily in dilute acids (even vinegar); decrepitates under high heat; generally stable to light
Heat sensitivity	Decrepitates (cracks apart with crackling) at high temperature; keep away from torches and steam
Care and cleaning	Cool soapy water only; never ultrasonic single crystals (risky even for marble); never steam; keep away from acids, vinegar and harsh household chemicals
Birthstones	None
Availability	Plentiful in every form; onyx marble in large carving blocks
Recognition	Recognised through doubling seen through clear rhombs, the acid fizz test taught in every classroom, and honey-banded onyx marble

Varieties (3)

Banded Calcite (Onyx Marble)

Colour	banded white, cream, honey, orange, red-brown and green-grey in parallel layers
Transparency	opaque (translucent at thin edges)
Clarity type	n/a (opaque)
Phenomena	rare chatoyancy in fibrous satin-spar calcite
Appearance	Translucent banded stone with flowing cream-to-honey stripes; takes a soft waxy-greasy polish and glows warmly when back-lit at thin edges.
Trade names	onyx marble • Mexican onyx • cave onyx • honey onyx • Oriental alabaster
Misnomers	Mexican onyx and Oriental alabaster (it is neither true onyx nor gypsum alabaster) • Mexican jade when dyed green • 'onyx agate' mislabeling
Typical sizes	very large blocks; slabs, urns and beads all common
Cutting styles	Beads, cabochons, carvings, bowls, vases and cameos; dyed slices for souvenir jewellery
Key separations	vs banded chalcedony (true onyx): calcite spot RI blinks to 1.66 and is scratched by a knife (H 3 vs 7); acid test effervesces; vs dyed howlite/magnesite: SG and band structure differ, dye pools in howlite's veins; vs coral: banded parallel structure vs coral's curved pore structure
Sources (localities)	Mexico (Puebla, Tecali - classic) • Turkey • Pakistan • Iran • Egypt • USA (Arizona, California) • Italy • Spain • China • Brazil
Availability	Plentiful and inexpensive; a staple of tourist and artisan jewellery
Market notes	Dyed blue/green 'onyx' slices are ubiquitous; natural honey banding needs no disclosure, dye does
Care notes	Scratches easily; keep from acids (even citrus and perfume); cool soapy water only
In short	Onyx marble is banded calcite: warm cream-and-honey stripes, soft polish, big sizes and a rock-bottom price. Hardness 3, effervescence in acid and aggregate RI blink separate it instantly from true banded chalcedony onyx. Usually dyed for vivid souvenirs - disclose treatment.

Size (ct)	USD per carat	Tier	As of
1.00–30.00	$0.20 – $3.00	commercial	2026-10-26

Retail market survey, per carat unless noted. Ranges describe what commercial to fine stones trade at; exceptional stones and provenance premiums sit above them.

Iceland Spar

Colour	colourless, water-clear
Transparency	transparent
Clarity type	Often remarkably clean in cleavage rhombs; faceted stones rare
Phenomena	none
Appearance	Glassy, perfectly clear cleavage rhombs that split every image into two; sold as specimens, teaching tools and the occasional novelty pendant.
Trade names	Iceland spar • optical calcite • double-refraction spar (Viking 'sunstone' legend is contested)
Misnomers	'Viking sunstone' (a popular-science claim, not established gem nomenclature)
Typical sizes	cleavage rhombs from a few cm to large museum pieces
Cutting styles	Natural cleavage rhombs; rarely faceted for collectors (soft, perfect cleavage)
Key separations	vs colourless quartz: extreme doubling, RI pair 1.486/1.658, hardness 3 and rhombohedral cleavage; vs glass: DR blink on polariscope and the famous double image
Sources (localities)	Iceland (Helgustadir - historic) • Mexico • Brazil • USA (South Dakota) • Russia • China • Spain • England (Cumbria)
Availability	Available as mineral specimens; faceted gems a curiosity
Market notes	Specimen and educational market; the historic Helgustadir mine is a protected reserve
Care notes	Handle gently - cleaves at a touch; no ultrasonic, no acids
In short	Iceland spar is water-clear optical calcite whose 0.172 birefringence doubles every image through the stone. Sold as cleavage rhombs and teaching specimens rather than cut gems; hardness 3 and perfect cleavage rule out wear. Iceland's Helgustadir mine is the historic source.

Size (ct)	USD per carat	Tier	As of
1.00–50.00	$0.50 – $8.00	specimen/novelty	2026-10-26

Retail market survey, per carat unless noted. Ranges describe what commercial to fine stones trade at; exceptional stones and provenance premiums sit above them.

Grey-Black Calcite

Colour	grey to black massive calcite, sometimes bituminous or with pyrite flecks
Transparency	opaque
Clarity type	n/a (opaque)
Phenomena	none
Appearance	Dark slate-grey to black massive stone, sometimes flecked with brassy pyrite; a soft, easily polished carving material with a dull-to-greasy lustre.
Trade names	black calcite • grey marble (when carbonate rock)
Misnomers	'black onyx' (usually dyed chalcedony, not this) • 'Belgian black marble' is a limestone, strictly a different rock
Typical sizes	large massive pieces
Cutting styles	Cabochons, beads, carvings, spheres and eggs
Key separations	vs black onyx: hardness 3 vs 7, RI blink, acid fizz; vs jet: SG 2.7 vs 1.3 and jet burns; vs black glass: acid test and aggregate blink; vs hematite: streak and SG
Sources (localities)	Widespread: Mexico • USA • England • Germany • Belgium (adjacent marbles) • China • Russia
Availability	Occasional in the trade, mostly as carved decorative pieces
Market notes	A minor ornamental material; verify species because 'black marble' trade terms blur limestone, calcite and dyed alternatives
Care notes	Soft and acid-sensitive - keep from citrus, vinegar and bleach; dust with a dry cloth
In short	Grey-black calcite is massive dark carbonate flecked with carbon or pyrite, cut as cabs, beads and carvings. Hardness 3, aggregate RI blink and instant acid effervescence identify it against black onyx, jet and black glass. A modest ornamental stone.

Size (ct)	USD per carat	Tier	As of
1.00–50.00	$0.20 – $2.00	commercial	2026-10-26

Retail market survey, per carat unless noted. Ranges describe what commercial to fine stones trade at; exceptional stones and provenance premiums sit above them.

Sources

GIA gem encyclopedia · mindat.org · Wikipedia

Data compiled and fact-checked against gemmological literature, October 2026 (JDMIS gem reference). Properties are ranges accepted in gemmological practice; individual stones vary.

Key properties

Varieties (3)

Banded Calcite (Onyx Marble)

Iceland Spar

Grey-Black Calcite

Sources

See also