Not as much people are familiar with red spinel, but almost the entire world knows about the majesty of ruby. Both of these gemstones can come in a brilliant scarlet red, reaching vivid saturation levels on a pure earth-mined and untreated condition. Of course, those would be the top qualities of these gem varieties, and thus would command a hefty price in the industry.
To those who have seen the rarer fine qualities of red spinel, you must already be familiar with its likeness to ruby. The general trade also prizes spinel for its brilliant colors and fine luster. Its beauty is the favorite of many gemstone connoisseurs around the globe- and up until the 19th century, a vast majority of the general public actually believed this red gemstone to be ruby itself.
The advent of gemology and a better understanding of crystals gave way to the separation between these two gem varieties (which also belong to different species). Here, we will take a quick look at some of their differences.
First of all, rubies are a variety of corundum. This gem species belongs to the trigonal crystal system, is doubly refractive and will show "doubling" under strong enough magnification.
Spinel is a another gem species altogether, which belongs to the cubic crystal system. It is singly refractive, and therefore will never show any kind of "doubling" under magnification.
Doubling refers to the visual "duplication" of back facets or inclusions when you look through the gemstone's interior, provided you are not looking down an optic axis direction.
A polariscope can help you separate ruby from red spinel simply by observing if your red stone "blinks" or not as you turn it full circle under crossed polars. Ruby will do this because it is an anisotropic stone (DR), while spinel will not.
Ruby's hardness as the mineral species corundum is also rated at a 9 on the Moh's scale, while spinel ranks at an 8. Both gems are very hard with optimum durability traits.
When mining for them, rubies are most commonly found as hexagonal tablet shapes, while spinels could resemble partial or whole octahedral shapes.
Their specific gravities are also different, with spinel ranging at 3.58 to 3.61 and ruby at 3.9 to 4.1.
Some people are able to heft this difference by bouncing gemstones on their palms, but the more accurate way to determine SG would be using a scale with a hydrostatic attachment. You could also use SG heavy liquids at your preference.
Natural, Untreated Rubies Will Always Command Higher Prices Compared to Spinels of Equivalent Color Intensity and Overall Quality.
Separating ruby and spinel can also be supported by using a refractometer. Spinel has an RI value of 1.719, and shows a singular value (therefore there is no birefringence observation). Ruby on the other hand, shows RI values of around 1.762 to 1.770. This simply means that if you view the refractometer's scale while turning a polarizing filter in front of the viewer, you will see that ruby's RI value will change upon a 90 degree rotation. This value will first be around 1.762, then change to 1.770 upon the turn.
Both red gemstones are extremely remarkable, and can show us specimens of undeniable beauty and earthly rarity. Many people will agree that while they can often look very similar, an untreated ruby will usually have a much higher price compared to a red spinel of its equivalent quality. This is true especially in the finest hierarchies of their color and clarity grades levels.
