Ceylonite and Pleonaste (first cited 1801) are dingy blue or grey to black varieties of spinel

Spinel

Spinel is the magnesium aluminium member of the larger spinel group of minerals. It has the formula MgAl2O4. Balas ruby is an old name for a rose-tinted variety.-Spinel group:...

. Ceylonite, apparently named for the island of Ceylon (where it is not known to occur), is a ferroan spinel with Mg:Fe from 3:1 and 1:1, and little or no ferric

Ferric

Ferric refers to iron-containing materials or compounds. In chemistry the term is reserved for iron with an oxidation number of +3, also denoted iron or Fe3+. On the other hand, ferrous refers to iron with oxidation number of +2, denoted iron or Fe2+...

 iron. Pleonaste is named from the Greek

Greek language

Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

for 'abundant,' for its many crystal forms, and is distinguished chemically by low Mg:Fe ratios of approximately 1:3.

Ceylonite and pleonaste are old terms with (apparently) no precise meaning. Nevertheless, both terms are still occasionally used in the current mineralogical literature.