Superb Hexagonal Ruby Crystal
Credit: JohnBetts-FineMinerals

Superb hexagonal ruby crystal, the red-pink gem variety of the mineral species corundum, in green zoisite matrix. The ruby crystal has exceptionally rich color and runs completely through the matrix. Longido, Arusha, Tanzania

Ruby is a gemstone version of the mineral Corundum, a simple aluminum oxide mineral. Corundum is rare on earth as it requires there to be lots of aluminum and a more limited amount of the most common component in the Earth’s crust; silica. On occasion, such as in metamorphic rocks of Tanzania, high-aluminum sediments have been heated enough to cause corundum to grow.

This piece of Corundum is showing you important details of its structure – there are bands where you can see the way the crystal grew outwards in stages and how the symmetry of the atomic positions imposes itself on every single layer, creating the hexagonal shape.

Aluminum doesn’t typically give minerals color as the electrons in that structure don’t absorb light in the visible range. For rubies to get their color, they have to have another component present with electrons that can absorb visible light. In the case of ruby, it is chromium; chromium has the same charge as aluminum and a similar atomic radius, so some of it can substitute into the ruby structure in place of the aluminum. The outer d-electrons in transition metals like chromium can commonly jump from one d-orbital to another using the energy available in visible light, causing them to absorb visible light and making them the pigments for crystals like this. The layering in this ruby is likely reflecting how much chromium was available at every stage in ruby growth.