Forsterite is the magnesium-rich endmember of the olivine solid solution series. It is frequently found as transparent yellow-green crystals in ultramafic rocks or thermally metamorphosed limestones, and is the primary mineral component of the gem variety known as Peridot.
Is this forsterite?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch forsterite with a known reference. Forsterite sits at Mohs 7, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: white.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: green, yellow-green, colorless, white, gray, yellow.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: crystalline, tabular.
Often confused with
Forsterite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside forsterite
Minerals reported to co-occur with forsterite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Mg₂SiO₄
- Mohs hardness
- 7
- Density
- 3.275 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Transparent to translucent
- Crystal system
- Orthorhombic
- Crystal habit
- Crystalline, tabular
- Cleavage
- {010}, {100}, imperfect
- Rarity
- Common
- Uses
- Collector, Gemstone, Refractory material
- Host rock
- In mafic and ultramafic igneous rocks and thermally metamorphosed impure dolomitic limestones
Where rockhounds find forsterite
Classic worldwide localities
- Peridot Mesa, four km southwest of San Carlos, Gila Co., and at Buell Park, Apache Co., Arizona, USA
- Monte Somma and Vesuvius, Campania, Italy
- Ojamo, Lohja, Finland
- Møre og Ramsdel, Norway
- Kovdor massif, Kola Peninsula, and at Zlatoust and Mt. Itkul, Ural Mountains, Russia
- Eifel district, Germany, at Forstberg
Field-hunting tip
Where it forms: In mafic and ultramafic igneous rocks and thermally metamorphosed impure dolomitic limestones. If you start seeing enstatite, plagioclase, phlogopite in float, you are in the right ground. Typical habit: crystalline, tabular. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify forsterite?+
Where is forsterite found?+
What rocks look like forsterite?+
What minerals are found with forsterite?+
What kind of rock does forsterite form in?+
What is forsterite used for?+
Find forsterite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
