So what exactly are Opal Doublets & Opal Triplets?
Well they aren't exactly to do with the number of opals but rather the setting. So to clarify:
An Opal Doublet is a slice of natural Opal that has had black stone backing fused to it. The backing is generally made up of onyx or ironstone matrix and it is attached to support the more fragile opal layer. The backing generally also improves the look of the opal making it appear darker & of higher quality, however in a reflected light a faint separation line can sometimes be seen.
An Opal Triplet is a step onward from the Opal Doublet...