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Chalk is a soft, white, powdery limestone consisting mainly of fossil shells of foraminifers
From old English cealc chalk, lime, plaster; pebble, from Greek khalix small pebble, in English transferred to the opaque, white, soft limestone
Fine Grained Rock, Opaque Rock
Gabbro is an intrusive igneous rock which is chemically equivalent to plutonic Basalt
Christian Leopold von Buch
From Latin glaber bare, smooth, bald
Coarse Grained Rock, Opaque Rock