Garnet

Garnet is not one mineral but a family of them, which is why it turns up in more colours than the deep red most people picture. The red comes from iron and manganese in the crystal structure rather than from anything added to it.

It reads near black in shadow and clearly red in direct light, so the stone changes depending on where you are standing, but the colour itself stays constant under any light source.

January's birthstone. Traditionally associated with constancy.