Appearance
Opal causes a gemmed or crystalline appearance. It allows gems and crystals to form anywhere on the body, or even turn part of the body into crystal (whether this restricts movement or not is up to you). Gems can be polished, faceted, or natural-shaped.
Opal can also cause an iridescent sheen through the coat, giving the design a nacreous appearance. This should not change the color of the design, it will just give it a pearly shine on top.
Opal is also able to affect horns and hooves, turning them gem-like or crystalline, in any color.
Opal can optionally turn any other mutations into a crystalline/gem variant.
