Answer: Opal
An opal is a gemstone, famous for its changing colours. It is particularly common in Australia. Opal displays "play-of-colour," which means that it flashes coloured light when turned in white light. This made it very valuable and appreciated in antiquity. Shakespeare talks about "opals" to describe the characteristics of Orsino. As he is changing and inconstant, he is compared to the opal, and how it flashes different, constantly changing colours.