fish eye?

[gretsch]

i saw a diamond where in some light it looked like there was a shadow circle within the stone, but in other light it wasn''t seen, it looked like a beautiful sunburst. how do i know how to look for a fish eye? can you tell by the specs of the diamond, or is it just something you need to know how to look for by sight?

 

[gretsch]

actually, it sounds more like a possible nail head than fish eye. is there any way to be sure?