Aragonite

CaCO3 - Mohs Hardness: 3 1/2 - 4.  Aragonite occurs as the hard skeletal material of certain fresh-water and marine invertebrate organisms, including pelecypods (e.g. Clams, oysters, mussels), gastropods (creatures such as snails, slugs, and whelks), and some corals. The accumulated debris from these skeletal remains can be thick and extensive, usually on shallow sea floors, and with time may transform into limestone.
Abstract/Description: CaCO3 - Mohs Hardness: 3 1/2 - 4. Aragonite occurs as the hard skeletal material of certain fresh-water and marine invertebrate organisms, including pelecypods (e.g. Clams, oysters, mussels), gastropods (creatures such as snails, slugs, and whelks), and some corals. The accumulated debris from these skeletal remains can be thick and extensive, usually on shallow sea floors, and with time may transform into limestone.
Subject(s): environmental sciences