well i measured the diameter.. just over 3" .. . and it is 3.5" tall last .5" is clear epoxy... so i'd suggest a slightly less than 3" x 3" volume of opal chips... was hard to take the right picture that showed how nicely they sparkle in the sun!!
my bue brought me 4 raw fire opal chips from Australia n its kinda a hit n miss with what you find inside when worked but if you have time n money it can be well worth it