Definition
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Etymology
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Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers called coal beds
From the Old English term col, which has meant mineral of fossilized carbon since the 13th century
Coarse Grained Rock, Fine Grained Rock, Medium Grained Rock, Opaque Rock
Travertine is a mineral consisting of layered calcium carbonate formed by deposition from spring waters
From Italian travertino a kind of building stone, from Tiburs, adjective from Tibur (Tivoli), in Italy
Durable Rock, Medium Hardness Rock
Fine Grained Rock, Opaque Rock