The axis of precious corals is inorganic minerals synthesized by a process known as biomineralization
. Precious corals take in calcium ions and bicarbonate ions, major
constituents dissolved in seawater, and form calcite, one form of
carbonate calcium (Table 1). A number of microscopic sclerites (Fig. 2)
and tree-like axes are both made of calcite.
The axis of precious corals is made into jewelry because of its
beautiful luster when polished, coming in the various shades of red,
pink and white. A red color component has been identified as a
carotenoid called canthaxanthin (Fig. 3). Carotenoids are among the
most abundant pigments in nature and found in vegetables such as
carrots, tomatoes and pumpkins as well as in animals such as feathers
of flamingos, salmons, trout and spiny lobsters. Meanwhile, the rigid
axis of reef-building corals is aragonite, a different form of
carbonate calcium (Table 1). Aragonite is too porous and rough to be
used as gemstones.
During biomineralization in precious corals, not the major element of
carbonate calcium but trace elements, mainly alkaline-earth metals such
as magnesium, strontium and barium as well as sulfate ions, are
incorporated into the axis from surrounding waters as impurities. The
amount of inorganic trace elements contained in the axis change
regularly in the growth process that occurs from the center outwards.
As a result, layers are formed in concentric circles resembling growth
rings (Fig. 4). The composition ratio of inorganic trace elements in
the axis also varies with habitats and seawater components. Looking at
Mg/Ca and Ba/Ca ratios in the precious coral axis from major habitats,
the composition ratios of them are different according to habitats and
each has values characteristic of its locality (Fig. 5). Trace elements
in precious corals are, therefore, expected to be used as important
scientific data indicating their habitats and harvested areas.
(Hiroshi Hasegawa of Kanazawa University)
Glossary
Biomineralization (Fig. 1): the process by which organisms form
minerals. Aggregates of inorganic minerals are referred to as “hard
tissues.” Familiar examples include corals, shells, pearls, bones,
teeth, and diatom earth.
[References]
●Hasegawa, H. and Yamada, M., 2010. Chemical analyses of carbonate
skeletons in precious corals. In: Iwasaki, N., ed., Biohistory of
Precious Coral: Corals Scientific Cultural and Historical Perspectives.
Tokai University Press, Hadano, Japan, pp. 43?60.
●Hasegawa, H., Rahman. M. A., Luan. N.T., Maki. T. and Iwasaki. N., 2012. Trace elements in Corallium spp. as indicators for origin and habitat. Journal of Experimental Marine Biology and Ecology, 414-415, 1?5.
●Iwasaki, N. and Suzuki, T., 2010. Biology of precious coral. In:
Iwasaki, N., ed., Biohistory of Precious Coral: Corals Scientific
Cultural and Historical Perspectives. Tokai University Press, Hadano,
Japan, pp. 3-25.
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