ew. piaskowa wyspa koralowa, piaszczysta/piaskowa wysepka koralowa

Dla odróżnienia od "motu".

If the accumulated sediments are predominantly sand, then the island is called a cay; if they are predominantly gravel, the island is called a motu.
http://en.wikipedia.org/wiki/Cay

Green Island (Wyspa Greena) – niewielka piaszczysta wyspa koralowa (ang. cay) o powierzchni...
http://tinyurl.com/ots2w7u
Ukształtowanie terenu, niskie wapienne lub piaszczyste wyspy koralowe.
http://tinyurl.com/njyt8bt

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Note added at 1 day3 hrs (2013-09-12 17:03:28 GMT)
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Wyspy koralowe z definicji są "zbudowane z wapieni rafowych", bo koral to wapień.

Koral – szkielety węglanowe [głównie węglan wapnia, czyli wapień] koralowców. (...) Korale są pochodzenia organicznego. Są to szkielety wapienne koralowców.
https://pl.wikipedia.org/wiki/Koral

Czyli każda wyspa koralowa jest wapienna - natomiast proponuję w tłumaczeniu odróżnić "cay" od "motu".

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Note added at 1 day3 hrs (2013-09-12 17:22:24 GMT)
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cay
a small low island or bank composed of sand and coral fragments, esp in the Caribbean area. Also called: key
http://www.collinsdictionary.com/dictionary/english/cay

cay, also spelled Key, small, low island, usually sandy, situated on a coral reef platform. Such islands are commonly referred to as keys in Florida and parts of the Caribbean. Sand cays are usually built on the edge of the coral platform, opposite the direction from which the prevailing winds blow. Debris broken from the reef is swept across the platform at high tide but is prevented from washing over the edge by waves produced by the refraction and convergence of waves around the platform itself. The accumulation of sand may at first move around but gradually will become stabilized as beach rock (sand and debris cemented at water level by precipitated calcium carbonate) is formed...
http://www.britannica.com/EBchecked/topic/100777/cay

Czyli kwestia piasku jest tu dość zasadnicza.