Coral is a hard substance that is made when tiny sea animals build protective outer shells (similar to the hard shells that protect snails). These shells are made of calcium, the same mineral that makes bones and teeth strong. When many of these small animals live and die in the same place, their shells build up to create large structures in the ocean.
The red coral (Corallium rubrum) that grows in the Mediterranean Sea and Red Sea has been especially valuable throughout history. People have used this red coral to make jewelry and medicine. While alive, it appears green and shrublike, resembling an underwater plant. It looks this way because the tiny coral animals attach themselves to one spot and cannot move around. Once out of water, it turns hard and red.
In ancient times, coral was used as currency, alongside gems, pearls, and gold. Some Bible scholars think that when the book of Lamentations 4:7 mentions something red and precious, it is talking about pearls instead of coral. However, Job 28:18 and Ezekiel 27:16 likely refer to red coral.