Chalcedony is a cryptocrystalline variety of quartz, which means that it is formed by aggregates of very tiny crystals. Those aggregates are the ones that, by forming layers, constitute the geometry of our crystal. As any medium, Chalcedony has a stronger refractive index than that of the air. This means that the light is slowed down when it enters the crystal, and it does so differently depending on the wavelength.
The consequence is that the different colours that compose white light are separated, and each one of them enters the next layer at a different point, which further separates the colours. Eventually, when they reach the end of the crystal, they reflect at the bottom with different angles. Provided the angle of total reflection, each of the colours will show up at a different point in the surface; generating a rainbow.
So there it is, colourful chalcedonies may not exist, yet chalcedonies full of colours do. One only needs to look closely enough.
Author: Sergi Batlle