Selenite Identification Guide

What Selenite Looks Like

Selenite is the transparent to translucent crystalline variety of gypsum (hydrated calcium sulfate). It is typically colorless, white, or pale gray, with a vitreous to pearly or silky luster. Crystals are often tabular, bladed, or prismatic, and large "satin spar" varieties show a fibrous, silky chatoyance. A hallmark is its perfect cleavage, which lets it split into thin, flexible, transparent sheets. Some specimens form sharp clear crystals (like the famous giant crystals of Naica, Mexico), others form rosettes (desert rose) or fibrous masses.

Step-by-Step Field ID Checklist

1. Try a fingernail scratch — selenite is Mohs 2, soft enough to scratch with a fingernail. This is the single best test.
2. Look for sheet cleavage — it peels into thin, slightly flexible transparent flakes.
3. Check luster — glassy on cleavage faces, silky/satiny in fibrous satin spar.
4. Note transparency — clear to translucent; you can often see through thin pieces.
5. Feel the weight — light for its size (low density).
6. Avoid water — selenite is slightly water-soluble; don't wash it.

Key Diagnostic Tests

• Mohs hardness: 2 — fingernail scratches it; defining property.
• Cleavage: one perfect cleavage (plus two others), yielding thin flexible sheets.
• Streak: white.
• Density: low, ~2.3 g/cm³ — feels light.
• Water solubility: slightly soluble; surface dulls or etches in water (handle dry).
• Acid: does not fizz in dilute HCl (distinguishes it from calcite, which does).
• Flame/heat: dehydrates and turns chalky white if heated.

Common Look-Alikes and How to Tell Them Apart

• Calcite (Iceland spar): also clear, but Mohs 3 (won't scratch with fingernail), fizzes in dilute acid, and shows strong double refraction. Selenite is softer and acid-inert.
• Quartz (clear quartz): Mohs 7 — scratches glass and resists a knife; selenite is far softer. Easy to separate by hardness.
• Barite: heavier (high density), Mohs ~3–3.5, doesn't peel into flexible sheets.
• Muscovite mica: also peels into sheets but is harder (~2.5) and the flakes are elastic and springy; selenite flakes are flexible but not elastic.
• Halite (rock salt): cubic cleavage and salty taste; selenite cleaves into sheets and is tasteless.

Where Selenite Is Typically Found

Selenite forms in evaporite deposits and sedimentary basins where sulfate-rich water dries up: salt flats, clay beds, gypsum quarries, and caves. Famous localities include Naica, Mexico (giant crystals), the gypsum dunes of White Sands and Oklahoma's selenite crystal beds (USA), and many evaporite basins worldwide. Desert rose selenite forms in sandy arid soils.