Identification guide · Minerals
How to Identify Actinolite
Look for slender, bladed, or fibrous green to grayish-green crystals with a vitreous luster and a white streak. Hardness sits near 5 to 6, scratching glass but yielding to a steel file. Two cleavage directions meet at roughly 56 and 124 degrees, typical of amphiboles, and cross-fiber masses can feel silky. Actinolite is often confused with tremolite, which is paler and lower in iron, and with pyroxenes, which cleave at nearly right angles instead.
Actinolite identification checklist
| Color | Green, Dark green, Grayish green, Black | Look at a clean, dry surface in daylight. Color zoning, banding or spots are as diagnostic as the hue. |
|---|---|---|
| Luster | Vitreous to silky | Fresh surfaces shine like glass; a dull surface usually means weathering, so chip or wet a corner. |
| Transparency | Translucent to opaque | Hold a thin edge up to a bright light and note whether light passes through. |
| Mohs hardness | 5–6 | It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will. |
| Streak | White | Drag a corner across unglazed porcelain (the back of a bathroom tile) and check the powder color. |
| Cleavage | Perfect prismatic, two directions at ~56°/124° | |
| Fracture | Uneven to splintery | |
| Specific gravity | 3.0–3.2 | Heavier than it looks (specific gravity about 3.1). |
| Crystal system | Monoclinic |
Actinolite look-alikes and how to tell them apart
Both sit near Mohs 5–6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Actinolite vs Hornblende →
Both sit near Mohs 5–6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Actinolite vs Tremolite →
Both sit near Mohs 5–6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Actinolite vs Diopside →
Both sit near Mohs 5–6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead.
Actinolite identification FAQ
How can you tell if a rock is Actinolite?
Check the color (green, dark green, grayish green, and black), the vitreous to silky luster, a Mohs hardness of 5–6 and a white streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Actinolite?
Actinolite has a Mohs hardness of 5–6. It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will.
What is the difference between Actinolite and Hornblende?
Actinolite (Mohs 5–6, vitreous to silky luster) and Hornblende (Mohs 5–6, vitreous to silky luster) look alike, but hardness, streak and heft usually separate them. See the full Actinolite vs Hornblende comparison for a side-by-side table.
Is Actinolite worth anything?
Entry-level Actinolite 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 Actinolite?
Actinolite is most often green, dark green, grayish green, and black. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
