Identification guide · Minerals
How to Identify Atacamite
Atacamite ranges from bright emerald green to dark blackish-green, with a vitreous to adamantine luster on crystal faces and a pale green streak. It has a hardness of 3 to 3.5, perfect cleavage in one direction, and typically forms slender prismatic or acicular crystals, though botryoidal and granular masses are more common. It closely resembles malachite and brochantite in color, but atacamite lacks malachite's effervescence in dilute acid and its crystals tend to be more elongated and less banded than malachite's characteristic concentric growth.
Atacamite identification checklist
| Color | Bright green, Dark green | Look at a clean, dry surface in daylight. Color zoning, banding or spots are as diagnostic as the hue. |
|---|---|---|
| Luster | Vitreous to adamantine | 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 | 3–3.5 | A copper coin (Mohs 3.5) scratches it, but a fingernail does not. |
| Streak | Apple green | Drag a corner across unglazed porcelain (the back of a bathroom tile) and check the powder color. |
| Cleavage | Perfect, one direction | |
| Fracture | Conchoidal | Broken surfaces show smooth, curved, shell-like scoops. |
| Specific gravity | 3.75–3.77 | Heavier than it looks (specific gravity about 3.76). |
| Crystal system | Orthorhombic |
Extra tests for Atacamite
Glass check. Atacamite 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.
Atacamite look-alikes and how to tell them apart
Both sit near Mohs 3–3.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Atacamite vs Calcite →
Apophyllite is harder (Mohs 4.5–5) than Atacamite (Mohs 3–3.5), so Apophyllite will scratch Atacamite and not the other way round. A copper coin (Mohs 3.5) scratches it, but a fingernail does not. Atacamite vs Apophyllite →
Both sit near Mohs 3–3.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Atacamite vs Celestine →
Both sit near Mohs 3–3.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead.
Atacamite identification FAQ
How can you tell if a rock is Atacamite?
Check the color (bright green and dark green), the vitreous to adamantine luster, a Mohs hardness of 3–3.5 and a apple green streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Atacamite?
Atacamite has a Mohs hardness of 3–3.5. A copper coin (Mohs 3.5) scratches it, but a fingernail does not.
What is the difference between Atacamite and Calcite?
Atacamite (Mohs 3–3.5, vitreous to adamantine luster) and Calcite (Mohs 3, vitreous luster) look alike, but hardness, streak and heft usually separate them. See the full Atacamite vs Calcite comparison for a side-by-side table.
Is Atacamite worth anything?
Entry-level Atacamite 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 Atacamite?
Atacamite is most often bright green and dark green. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
