I realized that I haven’t geeked out over the gemology of birthstones in a while. So a slightly late visit to September’s traditional birthstone, sapphire.
Sapphires are a gemstone variant of corundum. Red gemstone corundums are known as rubies, every other color is categorized as sapphire. (Despite the very name coming from a Greek word for blue.)
For a long time the word sapphire was applied to different stones. Even into the Middle Ages a reference to sapphire was probably to what we now call lapis lazuli.
Corundums are composed of an aluminum oxide, the bond between the aluminum and the oxygen atoms are short and the atoms are nestled tightly together. This makes them comparatively dense despite being made of light elements, and it makes them durable. As mentioned with rubies, corundum is second only to diamond for hardness. The majority of corundum found is used for industrial purposes.
Because of its sturdiness corundum lasts long after other sediments wear away. Since it is dense, it tends to be found in placer deposits (where erosion and gravity have essentially sifted the heavier bits and pieces together into a gravel over the millennia) and alluvial deposits (where water washed away the original sediment).
For traditional blue sapphires, value is determined by the color of the stones-how pure, how vivid, and how dark. Generally color wise the bluer the better. The blue color comes from traces of titanium and iron.
A small amount of purple secondary hue is considered acceptable, and if subtle enough, can even raise the value of the gem. Vanadium is the coloring element that brings purple and violet (and orange!). Any trace of green (a smaller iron content) lowers it vastly. (I don’t know why, sounds like it’d be a lovely color to me!) Likewise, the richer and more saturated the color the better.
Depending on who you ask the most preferred shades are either an almost primary blue or a slightly lighter rare cornflower blue found in a small part of the Himalayas.
Traditionally sapphire has symbolized loyalty, faithfulness, sincerity and truth. The richness of its color keeps it associated with nobility and royalty. And despite its myriad of shades, it is first and foremost the blue to which every other gem is compared.