Coral Rock

Coral rock is a limestone composed largely of the calcium carbonate skeletons of corals together with other reef-building organisms such as coralline algae, mollusks, and foraminifera. It is essentially a fossilized or lithified coral reef.

The rock is typically porous and cavernous, with the branching and massive forms of the original corals often preserved. Colors are usually pale: white, cream, tan, and gray. It is the bedrock of many tropical islands and coastlines.

Widely known as keystone or coral stone in the building trade, coral rock has been quarried for construction in the Caribbean, Florida, and Pacific and Indian Ocean islands for centuries.

Coral rock forms in warm, shallow, clear tropical seas where reef-building corals and associated organisms secrete calcium carbonate skeletons. As the reef grows and older parts die, their skeletons accumulate and, with coralline algae and shell debris, build a rigid carbonate framework.

Over time this framework is cemented, compacted, and sometimes uplifted above sea level, producing solid coral limestone. Many low tropical islands and raised reef terraces are made entirely of such rock.

Fresh-water percolation can recrystallize and further harden the rock, and continued reef growth keeps pace with subsiding sea floors to build thick carbonate platforms over geological time.

Coral rock is recognized by its pale color, high porosity, and the visible fossil coral structures, including branching, brain-like, and honeycomb patterns, preserved in the stone.

As a carbonate rock it fizzes in dilute hydrochloric acid and is fairly soft at around Mohs 3 to 4. It often feels rough and pitted, with cavities and molds where coral and shells dissolved away.

Look-alikes include other reef and shelly limestones; the distinctive preserved coral skeletons are the key clue. It should not be confused with precious red or pink coral used in jewelry, which is unlithified coral skeleton, not a rock.

Coral rock has been a traditional building stone in tropical regions, cut into blocks for houses, forts, churches, and seawalls in the Caribbean, Florida (where it is called keystone or coral stone), Bermuda, and many island nations.

It is also crushed for aggregate, road base, and to make lime and cement, and porous varieties are used in landscaping and aquariums. Polished fossil coral can be made into ornamental stone and cabochons.

Because modern reef coral is ecologically protected, much building coral rock today comes from ancient, long-dead raised reef deposits rather than living reefs, and harvesting living coral is widely restricted.