Garnet Varieties: Gemstone Information
The word ‘garnet’ is known from antiquity and is derived from the Latin word granatum – pomegranate, which in turn derives from granum – grain; meaning multi-seeded in the context of the pomegranate, the seeds of which turn a deep wine-red when ripe. The open fruit shows similar coloured seeds and surrounding pith, thus the allusion to the resemblance with the mineral. The name garnet is applied to a complex group of minerals that have common crystal structure and some similarity in chemical composition.
Garnets Varieties present a complex classification problem, since their chemical composition is a continuous admixture of end-member components, with few (if any) natural breaks that can be used to distinguish among gemmologically significant varieties. Varieties have been defined (Stockton and Manson) as subdivisions of a species that may be differentiated either by distinctive physical characteristics such as colour and phenomena or by consistent minor chemical disparity; however, some subjectivity is still prevalent and this gives rise to uncertainty. They have similar crystal lattice structure (isostructural) and can, with the exception of the hydrogarnets (hibschite and katoite), be expressed by the general formula A3B2(SiO4)3. Within the garnet group, the mineral species approved by the International Mineralogical Association’s Commission on New Minerals and Mineral Names (CNMMN) are:
Hibschite Ca3Al2(SiO4)3-x(OH)4x (where x is between 0.2 and 1.5)
Katoite Ca3Al2(SiO4)3-x(OH)4x (where x is greater than 1.5)
Schorlomite Ca3Ti24+Fe2 3+Si)O12
(A site replacement element)
(B site replacement element)