Limestone is a fine to medium-grained sedimentary rock. It is usually grey, white, yellowish or tan. It can often be streaked red with iron or black with magnesium impurities. Limestone is made up of the mineral calcite, which fizzes in white vinegar.
It does not contain any visible crystals but it can sometimes look sugary.
Limestone typically contains fossil shells and corals which are themselves made up of calcite.
How is it formed?
Limestone can be formed by the burial of coral reefs, but mostly it forms from lime rich mud in warm tropical waters.
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