Up until the 19th century, ruby and red spinel were thought to be the same mineral and with good cause. The chemical composition, color, hardness, and general appearance of a red spinel can be deceptively similar to ruby. Without special equipment, it is difficult to tell them apart. However, one major difference between the two is that ruby is doubly refractive while spinel is singly refractive.
Left – Very fine ‘pigeon blood’ Mozambique Ruby displaying violet fluorescence and silk. Right – Average quality spinel from Vietnam
What does that mean?
When light enters a doubly refractive gem, the light is split in two with each beam traveling at a different speed. The difference in speed in the two beams is known as “birefringence.” The bigger the birefringence, the easier it is to detect that a stone is doubly refractive. In some highly doubly refractive stones, like zircon, you will feel like you’re seeing double when looking into the stone – you will see two sets of facet junctions and internal characteristics.
All corundum (sapphires and rubies) are doubly refractive but with a relatively low birefringence. Herein lies the reason why the ancients always considered spinel and ruby to be the same – they did not have the technology needed to detect this small difference in light speed.
Today, it is relatively easy to detect double refraction. Using a polarizing filter, double refraction is clearly seen by turning the filter 90 degrees.
In my opinion, spinels have long been the ‘underdog’ of gems because of its constant comparison to ruby (this post included!)
Bearing the reputation of having been the culprit in a case of mistaken identity involving a large and famous crown jewel passed through the hands of many a royalty, spinel has never recovered.
Some spinels are truly spectacular in their own right. Like a special shade of neon-like red called ‘open red’ (the most desirable of the red shades) and a shade of neon pink with a tinge of orange.
Left – ‘open red’ spinel. Right – neon pink with orange spinel
Compare the above spinels (here we go again, with the comparisons!) with a finer color ruby on the market below and you’ll see why spinels shouldn’t be overlooked! Pound for pound, in terms of color, a spinel can rival that of a much more costly ruby.