Most coral reefs are made up of stony corals, but blue corals, organpipe corals, and precious red corals also make up some reefs because they too have a stony skeleton. The corals themselves are only 1/2 inch in diameter. These corals grow together to form colonies about the size of a small car. Then these colonies join with other colonies to form a reef. The largest coral reef is the Great Barrier Reef. This reef is located off the east coast of Australia and spans 1,600 miles. By looking at the yearly growth rings within the corals at this reef, it has been growing about 20,000 years. This in itself shows that corals grow very slowly, most less than an inch per year. Individual colonies can live decades to centuries. Corals spawn by asexual and sexual reproduction. Most of the corals that create the reefs, like stony corals, use external sexual reproduction. This often only occurs once per year, but is sometimes synchronized for all individuals of the same species within the area.