You can get pretty excited about gemstones without even leaving the sapphire family. Most people think that sapphire is available in blue and shades of blue.That is true, but not exclusively. Sapphire is available in a wide range of colors. When it is red, it's known as ruby.
Most colors fall between those shown below.

Oval Blue Sapphire
Cushion Cut Ruby
Cushion Pink Sapphire
Oval Padparascha Sapphire

Dichroism

Corundum is a dichroic stone. Because of its hexagonal structure, the corundum crystal actually absorbs different wavelengths, depending on which axis the light is propogating through. Independent of trace elements, corundum shows two different colors (at various saturations) when it is faceted.

corundum crystalRough sapphire crystals showing the hexagonal structure

Corundum: The Sapphires and Ruby

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