Gabbro

Gabbro is a coarse-grained, mafic intrusive rock that forms when magma cools slowly underground. It’s composed mainly of calcic plagioclase feldspar (such as labradorite or bytownite), along with pyroxene and iron oxides.

When the rock becomes more than 90% plagioclase, it’s called anorthosite.

One type of decorative gabbro, known as Indigo Gabbro, shows a blue iridescence when the feldspar catches the light.