Dull dark grey rock, which flashes peacock blue or green when turned to be at the right angle to the light. These two photographs show the same specimen at different angles.
Feldspars can show interesting optical effects, but labradorite is the most splendid.
The colours are a reflection of the light, broken up to cause the colour. However, a rough piece of labradorite can show small glints of colour as you turn it, as small pieces of the stone reflect light at the right angle.