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Selenite vs Calcite: How to Tell Gypsum From Calcite at Home

By Digital Towns Crystals · Last reviewed October 8, 2026

Short answer: selenite is a clear variety of gypsum, hydrous calcium sulfate at Mohs 2; calcite is calcium carbonate at Mohs 3. Your fingernail scratches selenite but not calcite. A drop of acid fizzes on calcite and does nothing to gypsum. Calcite breaks into rhombs and doubles print seen through a clear piece; selenite peels into thin, slightly flexible sheets. At equal size, calcite is about 17 percent heavier.

Why are selenite and calcite so easy to confuse?

Both are soft, common, colorless to white minerals with a glassy to pearly luster, and both turn up in the same places in a shop: as clear blades, white towers, polished spheres and carved bowls. Both also grow in sedimentary settings, sometimes side by side. Geology.com lists calcite among the minerals gypsum is commonly found with in layered evaporite deposits. A tray of unlabeled white crystals at a show can easily hold both.

The confusion matters because the two need different handling. Gypsum is water soluble and very heat sensitive, according to the International Gem Society, while calcite tolerates a rinse but dissolves in acids. Cleaning one by the other's rules can damage it.

Selenite and calcite side by side

Property Selenite (gypsum) Calcite
Chemistry CaSO4·2H2O, a hydrous sulfate CaCO3, a carbonate
Crystal system Monoclinic Trigonal
Mohs hardness 2 (as low as 1.5 in some directions) 3
Specific gravity 2.3 2.71
Cleavage One perfect, peels into thin sheets Three perfect, rhombohedral
Cold dilute acid No reaction Strong fizzing
Birefringence 0.009 to 0.010 0.154 to 0.174
Water Slowly soluble Tolerates a brief rinse
Common trade forms Blades, fishtail twins, ram's horns, spheres, lamps Rhombs, optical pieces, towers, spheres, palm stones

Hardness and gravity come from Geology.com's gypsum and calcite pages and webmineral; the birefringence figures are webmineral's for gypsum and calcite. That last row is the reason one doubles print and the other does not: calcite splits light into two rays roughly seventeen times more strongly than gypsum.

Six tests that separate them

None of these needs lab equipment. Do them in roughly this order, from harmless to slightly destructive.

1. The fingernail

Geology.com's Mohs article puts a fingernail at 2 to 2.5 and a copper sheet at 3. Press a fingernail firmly on an inconspicuous edge and drag. Selenite takes a groove; calcite does not. A copper coin, at about 3, will barely mark calcite but plows into gypsum. On a polished sphere, do this on the base contact point, never on the display face.

2. A drop of acid

Calcite is the textbook carbonate. In Geology.com's acid test table, calcite shows a strong reaction to cold dilute hydrochloric acid, while gypsum, a sulfate, releases no carbon dioxide at all. Without lab acid, household vinegar works: Geology.com describes vinegar as dilute acetic acid that gives a very weak reaction with calcite, best seen with a hand lens. Put one drop on a hidden spot, watch for tiny bubbles, then rinse and dry. Do not do this on a polished face, since the acid etches calcite.

3. How it breaks

Look at chipped edges or the ends of raw pieces. Calcite cleaves in three directions into rhomb-shaped blocks with flat, mirror-like faces meeting at oblique angles. Selenite has one dominant perfect cleavage; thin flakes peel off like mica and, as Minerals.net notes, slim gypsum crystals can be slightly bent before they break. Calcite never bends.

4. Double images

Lay a clear piece over a printed line. Transparent calcite shows two copies of the line, the effect that made Iceland spar famous; Minerals.net notes that most Iceland spar on the market now comes from Chihuahua, Mexico. Clear selenite shows one clean line. This test needs reasonably transparent material, so it works on optical calcite and window-clear selenite but not on milky towers.

5. Weight in the hand

With a specific gravity of 2.71 against about 2.3, calcite is roughly 17 percent heavier than gypsum of the same volume. You can use listed sizes to check: a 6 cm sphere has a volume of about 113 cubic centimeters, so it should weigh about 260 g in gypsum and about 306 g in calcite.

6. Under longwave UV

Calcite is commonly fluorescent, with specimens from different localities glowing different colors, according to Minerals.net. Gypsum glows less predictably: the International Gem Society says some gypsums, but not all, show fluorescence and phosphorescence. A glow therefore does not prove calcite on its own; treat UV as a supporting test after the fingernail and the acid drop.

How and where does each one form?

Gypsum is an evaporite. It precipitates as salty water evaporates, in layered sedimentary beds alongside halite, anhydrite and dolomite. Some of the most spectacular selenite on record comes from the mines at Naica, Chihuahua, which Minerals.net describes as Mexico's most notable locality and the site of caves with the world's largest known crystals. The International Gem Society lists Utah among many US sources and pictures a ram's horn selenite crystal from Guangdong Province, China.

Calcite forms in far more ways: from seawater as limestone, by metamorphism as marble, in caves as flowstone, and in veins and cavities as crystals. Geology.com calls it the principal constituent of limestone and marble. Collectors know it from the Elmwood Mine in Tennessee, which Minerals.net credits with very large, lustrous golden crystals, and from the Kansas, Missouri and Oklahoma tri-state district.

How do they compare on price and use?

Both are inexpensive at ordinary sizes because both are abundant. Gypsum is an industrial staple: the USGS states that most US gypsum goes into wallboard and that a typical new American home contains more than 7 metric tons of it. Calcite, as limestone and marble, is quarried in enormous volumes for construction and acid neutralization.

At the collector end, price follows clarity and form. Water-clear selenite blades, twinned fishtails and curled ram's horns command more than milky material. In calcite, transparent optical pieces that double images cleanly, saturated colors and well-formed crystals on matrix sit above massive banded or "onyx" material.

In crystal traditions, selenite is associated with clarity and is often made into plates and lamps; calcite's associations vary with its color. Practically, calcite is the better choice for something that will be handled or dusted with a damp cloth, while selenite belongs in a dry, low-traffic spot (see selenite care for the details).

Which should you buy?

  • For an optics demonstration, buy clear calcite: the double image is the most striking at-home mineral test there is.
  • For a large, luminous display piece on a budget, buy selenite or a satin spar carving (the satin spar comparison explains which is which).
  • For a polished shape that will be handled, buy calcite; at hardness 3 it still scratches, but it will not wear under a fingertip the way gypsum does.
  • For a beginner's hardness kit, buy one of each. They are minerals 2 and 3 on the Mohs scale itself.

Shopping selenite and calcite we stock

Use the listed dimensions and weights to run the gravity check before you buy, and ask for a fingernail test photo if a white piece is unlabeled.

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Selenite and calcite in stock

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Frequently asked questions

Is selenite a type of calcite?

No. Selenite is a variety of gypsum, a calcium sulfate with water in its structure. Calcite is calcium carbonate. They share calcium and a white color, but they belong to different mineral classes and react differently to acid and water.

Which is softer, selenite or calcite?

Selenite. Gypsum defines 2 on the Mohs scale and calcite defines 3, so a fingernail scratches selenite but not calcite, and calcite will scratch selenite. Glass and a steel knife blade, both above 4, scratch either one easily.

Does vinegar react with selenite?

No. Vinegar reacts with carbonates such as calcite, slowly releasing carbon dioxide bubbles. Gypsum is a sulfate and does not fizz. Avoid wetting selenite anyway, because gypsum is slowly water soluble and can lose its luster.

Why does my clear crystal show two images?

That is double refraction, and in a clear white stone it almost always means calcite. Gypsum is also doubly refractive, but about seventeen times more weakly, so the two images in selenite overlap and look like one.

Can calcite go in water when selenite cannot?

A brief rinse in plain water does calcite no harm, and it is the safer of the two to wipe clean. Avoid acidic cleaners, which etch it. Selenite should be dusted dry, since gypsum dissolves slowly in water.

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