Eclogite
Eclogite is a dense, coarse-grained metamorphic rock composed primarily of red garnet and green sodium-rich pyroxene called omphacite, formed only under the extreme pressures found deep within the crust or upper mantle. Its striking red-and-green coloration makes it one of the most visually distinctive metamorphic rocks, and its mineralogy records some of the highest-pressure conditions preserved in rocks that return to Earth's surface. Eclogite forms from basaltic protoliths subjected to pressures typically exceeding those of ordinary regional metamorphism.
Eclogite properties at a glance
| Type | Metamorphic Rocks |
|---|---|
| Mohs hardness | 6–7.5 |
| Luster | Vitreous (garnet and omphacite grains) |
| Fracture | Uneven |
| Specific gravity | 3.3–3.6 |
| Transparency | Opaque |
| Common colors | Green (omphacite), Red (garnet), Mottled green and red |
| Rarity | common |
Geology & background
Eclogite forms when basaltic oceanic crust is subducted to depths of 35 to well over 100 kilometers, where high pressure converts the original plagioclase and pyroxene into garnet and omphacitic pyroxene without the rock ever fully melting. Because these rocks require such extreme depths to form, their presence at the surface signals that a slice of subducted crust has been tectonically exhumed, a process documented at classic localities including the Western Gneiss Region of Norway, the Dabie-Sulu belt of China, the Franciscan Complex of California, and the Alps. Some eclogites also carry ultra-high-pressure indicator minerals like coesite or microdiamond, providing direct evidence of subduction to mantle depths.
Rockhounds should understand that eclogite is scientifically important as a window into deep subduction processes, even though intact outcrops are geographically limited and often heavily altered or retrogressed back toward greenschist-facies minerals during exhumation. Fresh, well-preserved eclogite with vivid red garnet embedded in bright green omphacite is considered a choice specimen for illustrating high-pressure metamorphism, and such material is far less common than typical regional metamorphic rocks like schist or gneiss.
How to identify Eclogite
Eclogite is identified by its coarse, granular texture and striking two-mineral color contrast: red to reddish-brown garnet grains set within a matrix of bright green to yellow-green omphacite pyroxene. It has an overall hardness reflecting its garnet and pyroxene content, roughly 6 to 7, no rock-scale cleavage but a granular fracture, and it lacks the strong foliation typical of schist or gneiss because eclogite generally forms without abundant mica. It can be mistaken for garnet-bearing amphibolite, but amphibolite contains black hornblende rather than green pyroxene, and eclogite's omphacite has a paler, more yellow-green color than amphibole's dark green to black tone.
- Compare the specimen's color, luster, crystal habit, grain size, streak, and locality before confirming.
- Use multiple photos when possible, including a fresh surface and a close view of texture.
Where Eclogite is found
Reported from multiple localities worldwide; verify locality against specimen provenance when available.
How much is Eclogite worth?
| Entry-level specimen | $1 |
|---|---|
| Typical collector specimen | $10 |
| Premium specimen | $75+ |
Value depends on size, crystal quality, color, locality, rarity, damage, and provenance.
Common uses
- Field identification
- Metamorphic history study
- Decorative or building stone when suitable
- Education
Interesting facts
- Some eclogites contain microscopic coesite or diamond, high-pressure minerals that prove the rock was once buried to depths of 100 kilometers or more before returning to the surface.
- Eclogite is denser than typical basalt or gabbro of the same composition, so when oceanic crust converts to eclogite during subduction it becomes negatively buoyant and helps drive the plate downward.
- The Western Gneiss Region of Norway hosts some of the largest known exposures of eclogite on Earth, offering geologists a rare surface view of deeply subducted continental crust.
Care & safety
Handle as a collector specimen until hardness, cleavage, and fracture are verified.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.
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