If you have precious opal atop either common opal or another substance, is it considered a doublet? No – not if they are formed naturally together.

Natural opal – natural precious opal cut to include common opal or potch in one piece
boulder opal - opal cut to include ironstone, rather than common opal or potch)
Doublet – A natural opal assembled with common opal, potch, or any other material
Triplet - the same.

Some dealers attempt to sell assembled opal and ironstone as boulder opal – don’t be fooled!

A wavy line between the two indicates a natural boulder opal. A straight line = an assembled boulder opal, i.e., a doublet.