Eclogite Identification Guide
Identify eclogite, the high-pressure red-and-green metamorphic rock, by its garnet-plus-omphacite mineralogy and how it differs from amphibolite and peridotite.
Read the full Eclogite encyclopedia entry →
What It Looks Like
Eclogite is a striking high-pressure metamorphic rock famous for its "Christmas" color scheme: rounded red to pinkish garnet crystals (pyrope-rich) set in a grass-green to apple-green matrix of the pyroxene omphacite. It is dense, granular, and usually lacks foliation, looking massive and granoblastic. Luster is vitreous on fresh grains. Accessory blue kyanite, white quartz, and rutile may be present, adding to the colorful look.
Telltale Visual Cues
- Red garnet "plums" in a green omphacite "pudding."
- Very high density — it feels heavy in the hand.
- Massive, non-layered texture (unlike schists and gneisses).
Step-by-Step Field-ID Checklist
- Spot the color pairing: red garnet plus green pyroxene is the defining signature.
- Heft it: eclogite is one of the densest common rocks (~3.4–3.6 g/cm³).
- Test garnet hardness: garnet is Mohs 7–7.5 and scratches glass; it shows no cleavage and conchoidal fracture.
- Examine the green matrix: omphacite is Mohs 5.5–6 with pyroxene cleavage at ~90°.
- Look for accessories: bright blue kyanite blades or red-brown rutile needles support an eclogite ID.
- Check texture: massive and granular, not foliated.
Key Diagnostic Tests
- Density: ~3.4–3.6 g/cm³ — distinctly heavy.
- Garnet: Mohs 7–7.5, no cleavage, conchoidal fracture.
- Omphacite: Mohs 5.5–6, two cleavages near 90°.
- Acid: inert to dilute HCl.
- Magnetism: generally nonmagnetic (minor if magnetite present).
- Streak: garnet white to pale; omphacite pale green-gray.
Common Look-Alikes
- Garnet amphibolite: also has red garnet but in a dark green-black hornblende matrix; hornblende shows cleavages near 120°/60° (vs pyroxene's ~90°) and the rock is often foliated.
- Garnet peridotite: green olivine matrix rather than omphacite; olivine has no good cleavage and the rock is more yellow-green.
- Garnet schist/gneiss: clearly foliated with micas, unlike massive eclogite.
- Rodingite: green but garnet here is grossular and the rock is lower density.
Where It Is Found
Eclogite forms at very high pressures — in subduction zones and the deep crust/upper mantle — and is exhumed in collisional mountain belts. Classic localities include the Western Gneiss Region of Norway, the Alps, the Dabie-Sulu belt of China, the Franciscan Complex of California, Scotland, and as xenoliths in kimberlite (sometimes diamond-bearing). Its presence indicates rocks once buried 45 km or deeper.
Frequently asked questions
What does eclogite look like?
It is a dense, massive rock with red to pink garnet crystals scattered through a bright green matrix of the pyroxene omphacite, sometimes with blue kyanite and red rutile accents.
What two minerals make up eclogite?
Eclogite is defined by pyrope-rich red garnet and green omphacite (a sodium-rich pyroxene). Quartz, kyanite, and rutile occur as accessories, but garnet plus omphacite is the essential pair.
How can you tell eclogite from amphibolite?
Both can carry red garnet, but eclogite's green matrix is omphacite (pyroxene, ~90° cleavage) while amphibolite's is hornblende (amphibole, ~120°/60° cleavage). Eclogite is denser and usually unfoliated.
Why is eclogite important to geologists?
It forms only at very high pressures in subduction zones and the deep crust, so finding eclogite at the surface signals that those rocks were once buried tens of kilometers deep and then exhumed.
Eclogite identified by the community
Recent Eclogite specimens identified with Rock Identifier.
