Fossilised coral

Most of the gem corals belong to the subclass Octocorallia (previously called Alcyonaria). The polyps of all octocorals have eight tentacles. Another subclass, Ceriantipatheria, includes the homy corals, and the subclass Zoantharia includes fossilised corals. 

Fossilised corals used today are of the orders Scleractinia, Rugosa and Tabulata. The laner two ate extinct. All three types of coral had... 

All living scleratinians have completely white skeletons, yet fossilised corals come in a variety of soft brown and pink- or yellow-brown colours. These colours are caused by geological processes, after the coral has died (Fig. 11.20). 

Fossilised corals lack a pattern of striations, but usually display small circles with radiating lines. Originally these circles would have been holes in the corals, but they are now infilled, and often silici-fied (Fig. 11.20). 

Fossil corals preserved in limestone are often seen in polished marbles used for ornamental or decorative ptirposes, but being relatively soft this material is less suitable for jewellery. Silicified fossil corals, being harder, are sometimes seen in jewellery. This type of material mostly comes from Antigua, India and China. Some fossilised material has earlier been found in Wiltshire, England, but this is now only seen in items such as old jewellery. 

Carbonate sedimentary rocks are dominantly of marine origin. Many limestones are composed of fossilised shell debris in various states of comminution, some fine grained carbonates such as chalk (q.v.) may be entirely composed of sub-microscopic phytoplankton. Algal mats secrete aragonite particles and coral reefs are CeiCO structures which may be preserved, fossilised, intact or degraded to produce other limestones. As discussed above, allochems such as faecal pellets and ooliths may be... 

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