Sometimes stones have natural inclusions that may look like fractures. The best way to determine it is to hold the stone up to the base of a lamp shade and roll the stone in your fingers keeping the stone in the light and not the shaded area. If there is a fracture you will see the light reflect off the fracture like a chip in your windscreen.
Natural or Synthetic stone?
Some synthetic or imitation opal can confuse customers. Usually they have no inclusions or potch backing behind them and the pattern appears to be spread evenly through the stone which is rare except for top gems. If viewed from the side the line of colour is at the lower areas. Some of these stones have an unnatural look and fire to them. It is best to become familiar with a few man made opals so it is easy to recognize them.