Black Eye On Calico Goldfish, Button eye Options

[goldfish8]

I have a calico ryukin with one black eye with black iris surrounding black pupil making the whole eye seem like one black spot while the other eye is normal with silver iris around the black pupil. I hear that this is an normal occurrence with calico goldfish. I have also seen videos on youtube where calico ryukins/goldfish have that one "black eye". Is this normal, rare, a problem? Why does this occur?

[bekko]

They call it "button eye". There is no reflective pigment in the iris. Metallic scale goldfish never have button eyes. Matt scale goldfish always have button eyes. Nacreous scale (calico) goldfish may have either button eyes or reflective eyes. It is not uncommon for them to have one button eye and one reflective eye. It is slightly less desirable to have one button eye and one reflective eye than to have matching eyes.

-steve