Clusters of light brown to darker brown corallites forming encrusting plates and mounds (hence Montastraea = Mountainous in the former scientific designation) that can spread over several tens of centimetres.

NOTE: The species was until recently known as Montastraea franksii. It is part of a group that includes another two variations known as Orbicella annularis (formerly Montastraea annularis) and Orbicella faveolata. All three belonged to the Faviidae family, but this has now also been renamed to Montastraeidae (in other words what used to be designation is now transferred to family name). This being said, the vernacular names in English and French have remained unchanged.  

Detailed descriptions in the little available literature on the Montastaelidae family (formely Faviidae) is scarce and often contradictory. However, it appears that the Boulder Star Coral distiguishes itself from the Great Star Coral (q.v.) in having corallites that form less less prominent cones (in other words with lower-gradient side walls). Coloration (light brown most common around Ilets Pigeon) and bleaching depend on the nature or absence of zooxanthellae.

Zooxanthellae are microscopic photosynthetic algae that live in symbiosis with corals. The latter provide shelter in exchange of which the zooxanthellae supply certain nutriments (including glucose and aminoacids relutling from photosynthesis) that enable them to eventually produce the calcium carbonate used to form their structure. When under stress - as happens with sudden temperature surges - coral expel their zooanthellae which causes them to lose their coloration in a phenomenon known as bleaching.