Brown to near-black, and much of what is sold has been irradiated to get there.
Every continent — the commonest mineral in the crust2 references
Smoky quartz gets its brown from natural radiation acting on trace aluminium in the crystal, over geological time. That process can be reproduced in an afternoon with a gamma source, and a large fraction of the very dark, very even smoky quartz on the market has been.
Treated material is not fake — it is quartz, and the colour is stable — but it is worth a great deal less than a natural crystal of the same depth, and a seller who does not disclose it is not being straight with you. The tell is the evenness. Natural smoky colour is usually zoned: stronger towards the termination, patchy, sometimes in visible phantom layers where growth paused. Irradiated stone tends to a flat, uniform, slightly grey-brown tone right through the piece.
Very dark, almost black smoky quartz is called morion, and near-opaque examples are more often treated than not. If a crystal is uniformly the colour of dark beer from base to tip with no zoning at all, assume treatment and price it accordingly.
Cairngorm · Morion (near-opaque black)
Establish which of these you are holding before judging it. A genuine piece read against another reference's rules will look wrong when it is not.
These apply to this model and not to the rest of Quartz. Work them alongside the maker's general checks, not instead of them.
Some cheap material is simply coated or heat-backed. Look at a chipped edge: the brown should run right through the fragment. Colour that stops at the surface is a coating, and a coating will wear off at contact points.
Hold the crystal to a strong light and look down its length. Natural smoky quartz almost always varies — denser at the tip, lighter at the base, or arranged in visible bands where the crystal grew in stages. Irradiated quartz is characteristically flat and even, because the dose was even.
Hardness 7, conchoidal fracture, no cleavage, no acid reaction, six-sided prisms with striated faces. Brown fluorite and brown calcite are both softer and both cleave; brown glass has bubbles.
Inexpensive. Natural well-formed crystals from named localities carry a premium over treated material of the same colour.
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.