Identification guide · Minerals
How to Identify Gypsum
Gypsum is soft enough to scratch with a fingernail, a defining hardness-2 property that immediately separates it from most other minerals, and shows a white streak and vitreous to pearly or silky luster depending on the variety. It has perfect cleavage in one direction, producing thin flexible sheets in the selenite variety, and colors range from colorless and white to gray, yellow, or brown. Habits include tabular crystals, fibrous satin spar, and massive alabaster. Gypsum is sometimes confused with calcite, but calcite is harder and reacts with acid, while gypsum does not.
Gypsum identification checklist
| Color | Colorless, White, Gray, Yellow, Brown, Pink | Look at a clean, dry surface in daylight. Color zoning, banding or spots are as diagnostic as the hue. |
|---|---|---|
| Luster | Vitreous, pearly, or silky (fibrous), earthy when massive | Fresh surfaces shine like glass; a dull surface usually means weathering, so chip or wet a corner. |
| Transparency | Transparent to translucent | Hold a thin edge up to a bright light and note whether light passes through. |
| Mohs hardness | 2 | A fingernail (Mohs 2.5) scratches it easily. |
| Streak | White | Drag a corner across unglazed porcelain (the back of a bathroom tile) and check the powder color. |
| Cleavage | Perfect, one direction; distinct in two others | |
| Fracture | Conchoidal to splintery/fibrous | Broken surfaces show smooth, curved, shell-like scoops. |
| Specific gravity | 2.3–2.35 | Ordinary rock-like heft (specific gravity about 2.325), similar to quartz. |
| Crystal system | Monoclinic |
Extra tests for Gypsum
Glass check. Gypsum shares a glassy luster and conchoidal fracture with man-made glass. Glass sits near Mohs 5.5 and often contains round gas bubbles, so a hardness test and a hand lens separate them.
Gypsum look-alikes and how to tell them apart
Anhydrite is harder (Mohs 3–3.5) than Gypsum (Mohs 2), so Anhydrite will scratch Gypsum and not the other way round. A fingernail (Mohs 2.5) scratches it easily. Gypsum vs Anhydrite →
Both sit near Mohs 2, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Gypsum vs Selenite →
Both sit near Mohs 2, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Gypsum vs Alabaster →
Calcite is harder (Mohs 3) than Gypsum (Mohs 2), so Calcite will scratch Gypsum and not the other way round. A fingernail (Mohs 2.5) scratches it easily. Gypsum vs Calcite →
Gypsum identification FAQ
How can you tell if a rock is Gypsum?
Check the color (colorless, white, gray, and yellow), the vitreous, pearly, or silky (fibrous), earthy when massive luster, a Mohs hardness of 2 and a white streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Gypsum?
Gypsum has a Mohs hardness of 2. A fingernail (Mohs 2.5) scratches it easily.
What is the difference between Gypsum and Anhydrite?
Gypsum (Mohs 2, vitreous, pearly, or silky (fibrous), earthy when massive luster) and Anhydrite (Mohs 3–3.5, vitreous, pearly on cleavage surfaces luster) look alike, but hardness, streak and heft usually separate them. See the full Gypsum vs Anhydrite comparison for a side-by-side table.
Is Gypsum worth anything?
Entry-level Gypsum specimens sell for around $2, typical collector pieces about $20, and premium examples $150 or more depending on size, quality and locality.
What color is Gypsum?
Gypsum is most often colorless, white, gray, and yellow. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
