Question: why are all crystals different colours?

Susana Teixeira answered on 26 Jun 2013:

They are not all different colours, it’s just that people tend to show the colourful ones more. Us crystallographers also tend to show photos of how crystals look when we use a type of light that highlights some directions in the crystal shape (it’s called polarised light, see the ones I show on my profile). This can make a transparent crystal look colourful/prettier 🙂

For crystals that have colour under normal light, this happens because of their ability to absorb some wavelengths in visible light and reflect the others. The specific colour depends on what type of molecule is there, and how it is distributed within the crystal. So for example in red rubies, the colour comes from a contamination of aluminium oxide crystal with chromium. Chromium sits in the crystal in positions that make it absorb light in the yellow-green region, so the entire crystals looks red.