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If you drip a drop of water onto a watch crystal you can tell if it is Mineral or Sapphire.

The bottom edge of the water drop (where it touches the crystal) will be FLAT on mineral, will be ROUNDED on Sapphire.

Water beads on Sapphire.

 

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It has been reported that FF act's the same as sapphire. The inconsistency you see with FF may be an indication that the coating is not uniform or that the you got one from the production run that is mineral (this has been known to happen from time to time).