These corals are known as ‘starred agate’ because their cross-section forms a star shape. They are a colonial coral, that is, they live together to form a structure much larger than the individuals that make it up. These are also reef building corals that today, form coral reefs. They are quite unlike solitary corals (see lower photo) that grow on hard surfaces such as bare rock. The corals were made of calcium carbonate, but this was replaced by silica during fossilisation.
Find out more on the corals page.