Peridotite Identification Guide

What Peridotite Looks Like

Peridotite is a coarse-grained ultramafic igneous rock made dominantly of olivine plus pyroxene, with little or no feldspar. Fresh surfaces are dark olive-green to greenish-black, often with a granular, sugary texture and a faint sparkle from pyroxene cleavage faces. Weathered surfaces turn rusty brown to reddish from oxidizing iron. It is notably heavy in the hand. Because it is so olivine-rich it can show patches of gem-quality green (peridot) and frequently appears partly altered to waxy, slick serpentine.

Step-by-Step Field ID Checklist

1. Assess color and weight. A very dark green to black, unusually dense, coarse rock points toward an ultramafic composition.
2. Look for green olivine grains. Glassy, sugary, olive-green crystals are the key mineral.
3. Check for a lack of feldspar. Little to no white or gray feldspar distinguishes it from gabbro.
4. Examine the weathering rind. A rusty-brown to orange crust over a green interior is typical.
5. Look for serpentine alteration. Slick, greasy green-black patches and veins are common.

Key Diagnostic Tests

• Hardness: Olivine and pyroxene are ~6.5–7; the rock scratches glass.
• Density: High, ~3.3 g/cm³ — heavier than granite or basalt of equal size.
• Grain size: Coarse, phaneritic (visible interlocking crystals).
• Acid: No reaction (unless calcite veins are present).
• Magnetism: Often weakly magnetic from accessory chromite or magnetite.

Common Look-Alikes and How to Tell Them Apart

• Gabbro: Also dark and coarse, but contains abundant gray-white plagioclase feldspar; peridotite has essentially none and is denser and greener.
• Dunite: A near-pure olivine peridotite — uniform green with almost no pyroxene; it's a subtype rather than a distinct rock.
• Serpentinite: The fully altered product of peridotite — soft (2.5–4), waxy, slick, and lighter; if a knife scratches it easily, it's serpentinite.
• Basalt: Fine-grained and dark; basalt lacks the coarse, sugary olivine crystals visible to the naked eye.
• Eclogite: Contains red garnet and green omphacite; brighter and more mottled than peridotite.

Where Peridotite Is Found

Peridotite makes up the Earth's upper mantle, so it surfaces where mantle material is exposed: ophiolite complexes (Oman, Cyprus, California's Coast Ranges), alpine peridotite massifs, layered intrusions (Bushveld, Stillwater), and as xenoliths carried up in basalt and kimberlite. Look in fault-emplaced ultramafic belts and serpentinite mélange zones.