The Spinel is a gemstone that is not well known in the commercial jewellery world, but one which has become a great favourite with gem dealers and gem collectors. Some spinels are among the most famous gemstones in the world, including the famous red gemstone in the centre of the British Crown Jewels. Before the advent of gemology as a science, spinels were called “Balas Rubies.”
Spinel is composed of magnesium aluminate, and coloured by chronium and iron. It occurs as octahedral crystals and has a complete absence of cleavage, unlike a diamond. Due to its excellent dispersion, spinel gemstones can possess vivid fire. This intensity of colour is partly due to the fact that spinel is singly refractive, where as most gemstones are doubly refractive. The best known singly refractive gems are diamond, spinel and garnet.
Spinel come in a huge variety of colour ranging from, white, black, purple, blue, green, red, pink, yellow, brown and orange. The deep-red variety, often called ruby spinel, is the most prized form, and then the hot pink and then flame orange. Any spinel gem weighing over two carats is rare.
Though ruby is slightly harder (9 on the Mohs scale), the spinel is hardness 8, it contains fewer inclusions than ruby, and spinel has greater fire and brilliance. Spinel is never heated nor treated in any way; indeed, there is no known treatment for improving the color or clarity of spinel. Conversely, virtually every affordable ruby is treated in some way these days. Also, since spinel is singly refractive and ruby is doubly refractive, the primary colour in red spinel appears purer and more intense than the red seen in many rubies. Yet, spinel is typically purchased for 20-50% the price of ruby. That makes the spinel tremendously good value on a market where prices for fine gems are rising every year.