Why Do Spinels and Rubies Sometimes Form Together?

You may be wondering how it is possible for spinels and rubies to be found together. Well, it is not surprising as they both come from the same metamorphic mineral (limestones, crystalline schists, gneisses, and serpentines). They are also present as a predominant mineral in rare mafic igneous rocks. So let's try to not be too complicated with details, and go straight to the point! Magmas in these igneous rocks are very poor in alkalis compared to aluminium. However, aluminium oxide may develop as a mineral corundum (to form rubies) or may be mixed with magnesia to form spinel.

Spinel petrogenesis in mafic magmatic rocks is still highly disputed but is likely the result of mafic magma encounters with more advanced magma or rock.

Fun fact: Two thousand years ago, gem traders believed that each and every bright red gem was a "ruby" and every soft blue gemstone was a "sapphire." As a result, many spinels are still in very significant jewelry collections based on their erroneous designation as ruby.

The ruby has a trigonal crystal structure, which implies it fluoresces in candlelight or daylight, but large stones are rare these days, while spinel has a cubic, sequentially refractive structure, comparable to a diamond, which leaves the stone bright and vivid. Rubies are aluminium minerals with traces of chromium that give them their red hue while spinels are magnesium aluminium minerals. Spinels can also be found in other tones, such as black, violet, and the very rare cobalt blue.

With over ten years of marketing experience, I finally decided to settle down in Bangkok to work in an area that I have always been interested in: the gemstones industry. I'm still learning how the market works on a daily basis, and I enjoy sharing my experience with those who are interested in learning more about gems in general.