How to distinguish Australian black Opal from all the other Opal types in the world. This Opal is what you call dark grey. A lot of people call it semi black in the black Opal group but you can see the certificate is not stating this and this is why you need a certificate when you go to most of the gem shows. A lot of the international gem testing Laboratories are very tough on grading black Opal as you can see in this video. The toughest of all would be the American Association. N grading system getting very big N1 to N10. This particular Opal would be a semi black graded N3 beautiful butterfly pattern. Most of the Opal sold in Australia as black Opal if tested by the biggest American gem testing Laboratory and Association would not be certified black Opal. So do your homework. These certificates when you're purchasing Opal as long as you see black group or you see grey. You are looking at a semi black or Opal in this grouping from jet black to black to dark grey. Most of these Opals can be classified in the black Opal group from N1 to N4.