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Summary
These "pearls" are non-nacrous and as such are not considered true pearls by gem experts, so they use the term "pearl" in quotation marks when referring to them. These beautiful gem-quality "pearls" come a gastropod mollusk called a melo melo snail.
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This snail inhabits the South China Sea, the waters around the Phillipines, the eastern coast of India, and the Andaman Sea. Countries like Taiwan, Malaysia, Indonesia and Vietnam harvest the large gastrapod for food.
Prevalence
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Melo melo "pearl" formation is most likely stimulated by an irritant on the top of its foot. These beautiful "accidents" are among the rarest gems on earth.
Colour
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The calcareous concretions this volute produces are colourful, non-nacreous, and very rare. Its "pearls" range from light tan to brown, but the orange is the most prized colour. Orange melo melo "pearls" can exhibit pronounced flame structure. Experts believe the colour of melo melo "pearls" is fairly unstable and prone to fading.
Size
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Melo melo "pearls" are usually spherical and quite large. One of the largest melo melo "pearls" examined weighs 397.52 cts. and is about three quarters the size of a golf ball.
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Article Source: http://www.twelvepearls.com.au/ (http://www.twelvepearls.com.au/)