Definition
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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
Claystone is a fine-grained, dark gray to pink sedimentary rock which mainly consists of compacted and hardened clay
From English clay and stone as the rock contains more amount of clay
Durable Rock, Medium Hardness Rock
Fine Grained Rock, Opaque Rock