Eclogite

Eclogite is a strikingly beautiful and geologically important high-pressure metamorphic rock. It consists mainly of crimson-red garnet (pyrope-rich) embedded in a matrix of grass-green omphacite, a sodium-rich clinopyroxene, often with accessory minerals like rutile, kyanite, or even diamond.

Its vivid red-and-green coloration is unmistakable and makes it a favorite of collectors. More importantly, eclogite forms only at very high pressures, recording conditions deep within the Earth, and is a key witness to subduction processes and the recycling of crust into the mantle.

Eclogite is comparatively rare at the surface, exposed in places like Norway, the Alps, and as xenoliths in kimberlite pipes.

Eclogite forms when basaltic or gabbroic rocks of the oceanic crust are subducted to great depths, typically more than 45 kilometers, where extreme pressure and moderate-to-high temperature transform them into the dense garnet-omphacite assemblage. The mineralogy is the high-pressure equivalent of basalt.

Some eclogites are exhumed back to the surface when subducted slabs rebound during continental collision, as in the Western Gneiss Region of Norway. Others are carried up from the mantle as xenoliths in kimberlite eruptions, sometimes containing diamonds. The remarkable density of eclogite (over 3.5 g/cm3) is thought to help drive the subduction of tectonic plates.

Eclogite is easily recognized by its distinctive appearance: round red to reddish-brown garnet crystals scattered through a bright to dull green pyroxene (omphacite) groundmass. It is notably dense and heavy in the hand.

The garnets are hard (around 7-7.5) and resist scratching, while the green matrix is slightly softer. Look-alikes include garnet amphibolite, but that contains hornblende rather than omphacite and is less dense; precise identification often needs thin-section study. Ruby-in-zoisite (anyolite) is superficially similar but contains black hornblende and pink-purple zoisite rather than green omphacite. The combination of red garnet, green pyroxene, and high density is diagnostic.

Eclogite is more valued for science than for commerce. It provides geologists with vital evidence about deep subduction, mantle composition, and plate tectonics, and diamond-bearing eclogite xenoliths are a source of natural diamonds.

Attractive specimens are popular among mineral and rock collectors for their vivid red-and-green colors. Occasionally the rock is cut and polished as a decorative or ornamental stone, and the garnets can be extracted as gems. It has no significant industrial use as bulk stone and little metaphysical following, its importance lying chiefly in what it reveals about the Earth's interior.