Boulder Opal or Opal Doublet?

A shop on etsy is offering opal jewelry, calling the items "boulder opals".  But what are they really?

I can see just by looking at them that these necklaces are actually "opal doublets" and NOT boulder opals at all.  What's the difference between a boulder opal and an opal doublet?

OPAL DOUBLET:

An opal doublet is a man-made opal.  This is a very inexpensive method for creating beautiful opal jewelry!  This gives the look of very expensive Black Opal.  A tiny slice of opal (about 1mm) is glued to a dark backing.  This backing can be made of industrial glass, onyx, hard plastic, ironstone, or even "potch" (common opal, or opal that doesn't exhibit the fire of precious opal).

You can see an opal doublet by simply looking at the side.  There is an obvious black backing and a straight line where it meets the opal.  This can be difficult to see when the opal is set.

It's important to NOT get an opal doublet wet, since the layers can separate!


BOULDER OPAL:

It is considered to be the second most valuable type of opal!  Ironstone boulders have thin veins of natural opal.  The stones are cut so the natural ironstone remains surrounding or backing these opal veins.  Ironstone is a dark brown stone, which makes the opal naturally darker and vivid.  Sometimes boulder opal is called a "natural doublet" since the ironstone is left behind the opal when cut for jewelry.  There is no adhesive involved in boulder opal!  Because the ironstone surrounds the opal naturally, it is much more durable than black opals.

If Boulder opals are cut to remove the opal for jewelry, they are always cut in irregular shapes, due to their natural veins within the ironstone.  They are never oval-shaped, except when the ironstone is left surrounding the opal (as shown here).  Boulder opals are also quite small, because the natural veins within the stone are thin.  

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On Etsy, there is at least one gemstone supplier who is offering opal doublets which are thin layers of opal that have been glued to ironstone.  This supplies seller clearly states that these are doublets.  It would appear that the jewelry seller on Etsy purchased these from the supplier (they look very very similar in shape and style).  If that's the case, then why is the Etsy jewelry seller (who knows these are doublets) calling these "boulder opals"?  I don't believe the seller is merely misinformed---I believe they are using "creative marketing" (i.e., fraud) to sell their jewelry.  These items are CLEARLY sold by the supplier as opal doublets and not "boulder opals"!  Plus, the seller can see the backing (which is conveniently NOT shown in the jewelry listings).  Boulder opals are natural and valuable opals, and doublets are much less expensive man-made opals.

The drilled opal doublet supplier on Etsy is called Gempalace, and their doublets are beautiful!!  (If you click on the link, you can see them!)  There is another doublet seller called  Casagemsbeads located in India, and theirs are beautiful as well, with more of a freeform shape.  Both of those gem suppliers are totally upfront and call their drilled opals "opal doublets".  I'm considering buying some from them myself!!

Just because an opal doublet is backed with ironstone (as opposed to onyx or glass) doesn't make it a boulder opal.  Those are TWO DIFFERENT THINGS, very different, and now you know the difference!

Buyer beware!