Flux-Grown Synthetic Red Spinel: Why Basic Tests Can Be Misleading

A synthetic spinel is grown by people but has the identity of spinel. It is different from a material that only imitates spinel’s appearance. Flux-grown red spinel is a useful example of why a correct mineral identification does not always establish natural origin.

Why routine measurements may overlap

SSEF’s 2009 study reported flux-grown red spinels with refractive index and density similar to natural spinel. Their attractive appearance and fluorescence were also insufficient for reliable separation. A familiar result on a basic gem test therefore did not settle the formation question.

What the researchers examined

Microscopy revealed cavities containing flux residues, sometimes with gas bubbles, and a metallic flake in one specimen. Chemical and spectroscopic observations helped support the interpretation.

Brown material inside a fissure must not automatically be labelled flux. Natural spinel can contain other brownish substances, and a similar-looking feature may have a different cause. Identification requires examining the feature in context.

A lesson for gem students

Separate the questions “Is this spinel?” and “How did this spinel form?” Basic tests may answer the first while leaving the second unresolved. When buying a stone described as natural red spinel, seek a report that addresses natural versus synthetic origin. A bright red colour, high clarity or a particular ultraviolet reaction cannot replace that conclusion.

Sources and further reading

SSEF Swiss Gemmological Institute, Facette, issue 16, January 2009, pp. 10, “Flux grown synthetic red spinels again on the market”. Read the original SSEF PDF.

This independent educational article explains historical SSEF research in original, simple English. It is not an SSEF publication. No source photographs or illustrations are reproduced.