Silicified fossils

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. 

A combination of several different opaline materials may be found juxtaposed. For example, in fossilized wood both tridymite and crystobalite may be determined in a sample whde adjacent portions of the sample may be composed of opal-A. The silicified wood may be transparent or translucent, clear and colorless, or white, yellow, red, brown, and black, indicating inclusions of other, usually iron-containing, complexes during precipitation of the colloid or gel. The faithfully preserved structures of fossilized wood suggest that the replacement phenomena are molecule-by-molecule processes that take place under low temperamres and pressures, and require concomitant removal of nonsiliceous compounds but do not disrupt the ceUularity of the woody tissues. Alternatively, primary... 

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