Information on Blue and Black Coral

by Manisha G

                                                              BLUE CORAL

This material, unusual in appearance and colour and of limited use, is obtained from the skeleton of Heliopera coerulea, a type of coral which lives mainly in the seas around the Philippines.

APPEARANCE

blue coralThe most striking characteristic of blue coral is its colour, which is bright blue or gray blue, sometimes with zoning in the form of concentric circles or even horizontal stripes. The most readily observable organic structures are two sets of channels parallel to the axis: one set is thin and barely visible through numerous channels. Due to the cavities, marks and discontinuities where the channels emerge, blue coral never takes a polish comparable to that of red coral. There­fore, its use in ornament is limited and mainly dependent on its colour. It is made into spherical, cylindrical, spindle or barrelshaped necklace beads, but does not lend itself to other uses.

DISTINCTIVE FEATURES

The colour and clearly visible organic structures makes it quite easy to recognize.

VALUE

Very low, partly because it is difficult to fashion and polish, it is essentially a curiosity on the western market

BLACK CORAL

This consists of the skeletons of polyp colonies mainly of the genera Gorgonia, Eunicella, Gerardia, and Parantipathes; but unlike those that make up red, pink and blue coral, these skeletal ramains are of a horny nature not calcic.

APPEARANCE

black-coralThe colour is black, but sometimes has minute, short, brownish yellow, slightly translucent streaks. It can acquire quite good luster if polished, but this will be of horny character, similar to that of some plastics. It is used in cylindrical pieces which are drilled along the axis or hori­zontally to it, as necklace beads. It can be bent, if heated and made into bangles. Cheap rings, carved items, and figurines up to ten centimeters tall are also made from larger pieces.

DISTINCTIVE FEATURES

If cut cross ways to the axis, the characteristic concentric rings, like those of tree trunks, are visible. These sometimes have marked discontinuities between one and the other, almost as though they were becoming detached. Faint radial structures and thin longitudinal structures, yellowish brown in colour and slightly translucent, may also be visible. Sometimes, these limited areas of yellow – brown translucency show small protuberances on the underlying surface, as would have been present on the outer surface had it not been polished (in black coral of the genus parantipathes). It is no use testing black coral with hydrochloric acid, as it does not contain calcium carbonate; but it should be remembered that the density is about 1.36g/cm3, which is much lower than that of pink corals. Due to its proteinaceous character, black coral emits a smell of burning horn if touched with a piece of red- hot wire. It is warm to the touch, like plastic, has a relatively low hardnes of between 2 and 3, and is slightly elastic.

VALUE

Distinctly low, much lower than that of the main types of the coral. As an ornament it is characteristic of many different cultures, but in the west it is mainly regarded as a curiosity, although appreciable quantities were seen on the market a few years ago.

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