Often hydrophane — it drinks water, and it takes dye and smoke treatment readily.
Australia (Lightning Ridge, Coober Pedy), Ethiopia, Mexico, Brazil
Ethiopian opal, principally from Welo, arrived on the market in quantity from the 2000s and changed the low and middle end of the trade. Much of it is hydrophane: porous enough to absorb water, which makes it temporarily transparent and can suppress the play-of-colour entirely until it dries out again. Some stones crack during that drying.
That porosity is also why Ethiopian opal takes treatment so well. Smoke treatment — heating the stone in organic matter so carbon deposits in the pores — darkens the body and makes the colour play read like black opal, and dyeing achieves similar ends. Neither is detectable by eye with confidence, and both are widespread enough that a dark Ethiopian stone at a modest price should be assumed treated.
The care implications are real and separate from authenticity. A hydrophane opal should not be submerged, cleaned ultrasonically, worn while swimming or bathing, or exposed to oils and lotions, all of which can be absorbed and are difficult to remove.
These apply to this model and not to the rest of Opal. Work them alongside the maker's general checks, not instead of them.
Smoke treatment and dyeing exploit hydrophane porosity to darken the body colour and imitate black opal. Neither is reliably visible, so the base rate is the evidence: expect treatment unless a seller states and can support otherwise.
Hydrophane opal turns transparent and loses colour play when wetted, which identifies it — but wetting the stone is exactly what should not be done to it, since some crack on drying. Ask the seller rather than experimenting.
Ethiopian material is more prone than most to developing a fine web of surface cracks as it loses water. Examine under a loupe with light raking across the surface, and treat any crazing as permanent.
Well below Australian material of comparable colour play, partly because of treatment and stability concerns.
An orientation for honest, unrestored condition — never an appraisal, and never a quotation.
Lovenhall StoneThe tests tell you what it is. The instrument reads a photograph and offers a name to start from — then hands you back to the tile and the coin, which are what settle it.