It's actually the hard, colorful calcareous skeleton secreted by certain marine polyps
In stony corals, also known as hard 
corals, the polyps produce a skeleton... [DOC] [TLE] Coral dictionary definition | coral defined [PAR] An example of coral is the pinkish-orange colored hard shell shaped like a twig 
used to ... the hard, stony skeleton secreted by certain marine polyps (class 
Anthozoa) and ... coral.