Identification guide · Minerals
How to Identify Forsterite
Forsterite-olivine typically forms as rounded, granular grains or short stubby crystals with a glassy, vitreous luster and colors ranging from pale yellow-green to deep olive green, with a white streak. Hardness is 6.5 to 7, and it has poor cleavage but a conchoidal fracture typical of many silicates. Transparent gem-quality material is called peridot. It is most often confused with green tourmaline or demantoid garnet, but olivine's lower birefringence, characteristic doubling of facet edges under magnification, and typical yellow-green hue help separate it from those look-alikes.
Forsterite identification checklist
| Color | Pale yellow-green, Olive green, Colorless | Look at a clean, dry surface in daylight. Color zoning, banding or spots are as diagnostic as the hue. |
|---|---|---|
| Luster | Vitreous | 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 | 6.5–7 | It scratches glass and a steel knife and cannot be scratched by a steel file. |
| Streak | White | Drag a corner across unglazed porcelain (the back of a bathroom tile) and check the powder color. |
| Cleavage | Poor | |
| Fracture | Conchoidal | Broken surfaces show smooth, curved, shell-like scoops. |
| Specific gravity | 3.2–3.3 | Heavier than it looks (specific gravity about 3.25). |
| Crystal system | Orthorhombic |
Extra tests for Forsterite
Glass check. Forsterite 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.
Forsterite look-alikes and how to tell them apart
Both sit near Mohs 6.5–7, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Forsterite vs Olivine →
Both sit near Mohs 6.5–7, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Forsterite vs Peridot →
Forsterite is harder (Mohs 6.5–7) than Calcite (Mohs 3), so Forsterite will scratch Calcite and not the other way round. A copper coin (Mohs 3.5) scratches it, but a fingernail does not. Forsterite vs Calcite →
Beryl is harder (Mohs 7.5–8) than Forsterite (Mohs 6.5–7), so Beryl will scratch Forsterite and not the other way round. It scratches glass and a steel knife and cannot be scratched by a steel file.
Forsterite identification FAQ
How can you tell if a rock is Forsterite?
Check the color (pale yellow-green, olive green, and colorless), the vitreous luster, a Mohs hardness of 6.5–7 and a white streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Forsterite?
Forsterite has a Mohs hardness of 6.5–7. It scratches glass and a steel knife and cannot be scratched by a steel file.
What is the difference between Forsterite and Olivine?
Forsterite (Mohs 6.5–7, vitreous luster) and Olivine (Mohs 6.5–7, vitreous luster) look alike, but hardness, streak and heft usually separate them. See the full Forsterite vs Olivine comparison for a side-by-side table.
Is Forsterite worth anything?
Entry-level Forsterite 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 Forsterite?
Forsterite is most often pale yellow-green, olive green, and colorless. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
