Science in Everything

So now we know that rubies and sapphires are the same mineral (corundum) and are differently colored because of different impurities.  Why should this be so?

Corundum is aluminum oxide (chemical formula Al2O3).  This means that for every two atoms of aluminum, there are three of oxygen.  These atoms are organized into what is called a lattice. The image below illustrates how a small piece of the lattice looks on an atomic level (gray = Al, red = O).

Pure corundum crystals would be colorless.  Like I said in my previous post, it’s chromium atoms that make rubies red and iron and titanium that make sapphires blue.

Chromium, iron, and titanium are what chemists call transition metals.  Aside from coloring gemstones, these metals can color fireworks and paint.  In rubies, some aluminum atoms are substituted with chromium atoms.  When light hits the mineral, red wavelengths are reflected, making it ruby red.

With blue sapphires, some aluminum atoms are substituted with titanium and iron.  These impurities “interact” in such a way that when light shines on the crystal, only blue wavelengths are reflected.

(Pictured above: aluminum oxide, ruby, sapphire)