Eclogite is an uncommon rock. It typically results from the high pressure metamorphism of mafic igneous rock such as basalt & gabbros at depths of 50km or more in the mantle or lower crust. It is composed of mainly two minerals, reddish-brown Almandine Pyrope garnet and the green sodium rich pyroxene Omphacite Other minerals, such as kyanite may also be present. Isotope dating of the Norwegian eclogites in the 1970s & 1980sindicates they are Early Palaeozoic (450-400 million years) and this suggests they formed during the Caledonian orogeny.
Studies have shown that western Norway had been dragged down to great depths beneath a thick pile of nappes that was building up as the mountain chain formed.
The discovery, about ten years ago, of micro-diamonds in eclogites was remarkable. They were too small to be seen with the naked eye, however, don't expect one in your sample. These examples come from Almenning, Norway. Specimens are supplied in labelled card trays.