Star sapphires

Can anyone explain to me how to determine if a sapphire is a
star sapphire (has asterism)? I have had great use of a lot of
the answers and comments from all of you experienced and creative
people out there and hope this question is not too much out of
the line with the orchid list. On a trip to down under a year ago
my wife and I went and digged for sapphires. We found quite a
few, some even of jewellery standard. These we had cut out there
over the night. However, the gem cutters said, that some of the
others were star sapphires, but now, where we are going to a
class in gem cutting, I cannot determine which are just black
carborundums (called bombs by the lokals) and which contaions
asterism. Any help would be much appreciated. Thanks.

from springtime Denmark, where the rain is pouring down