Identification guide · Minerals
How to Identify Gibbsite
Gibbsite is usually white, gray, or pale green to pale yellow with a dull to pearly luster and a white streak, occurring most often as earthy, chalky, or stalactitic masses rather than distinct crystals. Hardness is low, around 2.5 to 3.5, easily scratched with a fingernail or copper coin, and it shows perfect cleavage on the rare crystal faces that do form. When crystals appear, they are thin, tabular, and pseudohexagonal. Massive gibbsite is easily confused with other clay-like aluminum or iron hydroxide minerals such as boehmite or goethite without laboratory testing.
Gibbsite identification checklist
| Color | White, Gray, Greenish, Reddish | Look at a clean, dry surface in daylight. Color zoning, banding or spots are as diagnostic as the hue. |
|---|---|---|
| Luster | Vitreous, pearly on cleavage, dull when earthy | 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 | 2.5–3.5 | A copper coin (Mohs 3.5) scratches it, but a fingernail does not. |
| Streak | White | Drag a corner across unglazed porcelain (the back of a bathroom tile) and check the powder color. |
| Cleavage | Perfect basal {001} | |
| Fracture | Uneven | |
| Specific gravity | 2.3–2.4 | Ordinary rock-like heft (specific gravity about 2.3499999999999996), similar to quartz. |
| Crystal system | Monoclinic |
Gibbsite look-alikes and how to tell them apart
Both sit near Mohs 2.5–3.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Gibbsite vs Calcite →
Andalusite is harder (Mohs 6.5–7.5) than Gibbsite (Mohs 2.5–3.5), so Andalusite will scratch Gibbsite and not the other way round. A copper coin (Mohs 3.5) scratches it, but a fingernail does not. Gibbsite vs Andalusite →
Both sit near Mohs 2.5–3.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Gibbsite vs Celestine →
Both sit near Mohs 2.5–3.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead.
Gibbsite identification FAQ
How can you tell if a rock is Gibbsite?
Check the color (white, gray, greenish, and reddish), the vitreous, pearly on cleavage, dull when earthy luster, a Mohs hardness of 2.5–3.5 and a white streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Gibbsite?
Gibbsite has a Mohs hardness of 2.5–3.5. A copper coin (Mohs 3.5) scratches it, but a fingernail does not.
What is the difference between Gibbsite and Calcite?
Gibbsite (Mohs 2.5–3.5, vitreous, pearly on cleavage, dull when earthy luster) and Calcite (Mohs 3, vitreous luster) look alike, but hardness, streak and heft usually separate them. See the full Gibbsite vs Calcite comparison for a side-by-side table.
Is Gibbsite worth anything?
Entry-level Gibbsite 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 Gibbsite?
Gibbsite is most often white, gray, greenish, and reddish. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
