How Do Rubies and Sapphires Form?

What are Rubies and Sapphires?

Did you know that ruby and sapphire are actually the same mineral, corundum? Among natural minerals, only diamond has a greater hardness. Not surprisingly, when combined with fantastic colors, rubies and sapphires are some of the most highly desired gemstones.

So, if ruby and sapphire are the same mineral, why do we distinguish them? They share the same gemological properties with only one distinction: rubies are gem-quality red corundum. All other colors of gem-quality corundum are considered sapphires. Non-gem quality corundum, like that used commercially as an abrasive, is simply called corundum.

How Does Corundum Form?

Corundum (chemical formula Al₂O₃) crystalizes in slow-cooling igneous or metamorphic rocks. The slower the rock cools, the larger the crystal can grow.

Corundum won’t develop in just any igneous rock. The right ingredients must be present. In particular, the melt has to be rich in aluminum (Al) but devoid of silicon (Si), one of the most common elements in the Earth’s crust.

Metamorphic rocks are also an important source of corundum. The mineral forms in metamorphosed ancient sea beds, where aluminum-rich thermal waters ran through the rock. These marbles supply some…