Purple and yellow in one stone. Genuine material comes from one deposit; the rest is heated.
Brazil, Uruguay, Zambia, Madagascar, Arizona
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Ametrine is a single quartz crystal showing amethyst purple and citrine yellow in adjacent zones, and natural ametrine essentially comes from one commercial source: the Anahí mine in Bolivia. Everything else on the market is amethyst that has been partially heated, so that the heated end turned yellow and the shielded end stayed purple.
The natural material shows a sharp, straight boundary between the two colours, following a crystallographic plane, because the two zones grew under different conditions in the same crystal. Partially heated stone tends to a soft gradient, or a boundary whose position has nothing to do with the crystal's geometry — it corresponds to how the stone sat in the kiln.
This is a disclosure question rather than a fraud question in most cases. Heated ametrine is quartz, the colour is stable, and it is sold legitimately. What is not legitimate is charging Bolivian prices for it.
These apply to this model and not to the rest of Amethyst. Work them alongside the maker's general checks, not instead of them.
Natural ametrine has a crisp planar division that lines up with the crystal's facets and internal geometry. A soft gradient, or a boundary that cuts across the crystal at an arbitrary angle, indicates partial heat treatment.
Natural ametrine has one significant source, in Bolivia. A seller who cannot or will not name the deposit is selling heated amethyst, and it should be priced as heated amethyst.
Modest as a gem — a clean faceted stone runs tens of dollars a carat rather than hundreds. Bolivian provenance carries the premium.
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.