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
Gossan is intensely oxidized, weathered or decomposed rock, usually the upper and exposed part of an ore deposit or mineral vein.
From Cornish gossen from gos, blood from Old Cornish guit
Durable Rock, Medium Hardness Rock
Fine Grained Rock, Medium Grained Rock, Opaque Rock