Ceylonite, named for the island of Ceylon, is a ferroan spinel with Mg:Fe from 3:1 and 1:1, and little or no ferric iron.

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

This formula creates a face centered cubic Bravais lattice, with a space group of Fd3m.

This allows for the remaining octahedral site to be open for defects, causing a variety in structure and physical properties.

The ceylonite first discovered on the island of Ceylon, modern day Sri Lanka, was found imbedded in calcareous spar, and accompanied by pyrite and micas.