Gabbro is a coarse grained alkaline rock which forms when magma hardens slowly underground instead of erupting. It is most commonly composed of a plagioclase feldspar like labradorite or bytownite, a pyroxene and iron oxides.

When a gabbro becomes more than 90% plagioclase it's known as anorthite feldspar. 

One type of gem gabbro is Indigo gabbro where the feldspar in the stone shows a blue glow when moved about in light. 

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