Citrine Geode: Why Most Are Heat-Treated Amethyst

Almost every large citrine geode and citrine cathedral on the market started life as an amethyst geode and was heated until the purple turned yellow-orange.

This is not a rumour from a forum. GIA states that most of the citrine on the market is the result of heat treatment, and that producers typically use low-temperature heat treatment to turn amethyst — the violet variety of quartz — into citrine, with the hue of the original amethyst influencing the richness and saturation of the final yellow. GIA also describes natural citrine as exceptionally scarce.

So when you see a metre-tall geode with an orange interior on a cut base, the correct expectation is treated material. That does not make it fake, and it does not make it a bad object. It makes it a treated amethyst geode, which is what the label should say.

Citrine is the transparent, pale yellow to brownish orange variety of quartz, and GIA calls it the top-selling yellow-to-orange gem largely because it is attractive and affordable. The most desirable colour is a saturated yellow to reddish orange with no brownish cast.

Everything else about it is quartz. The U.S. Geological Survey lists citrine among the coarsely crystalline quartz varieties next to amethyst, rose quartz and smoky quartz, so it shares their hardness of 7 on the Mohs scale — it will not be scratched by a steel nail — and it breaks with the same curved conchoidal fracture rather than along flat cleavage planes.

There is also a naming history worth knowing: before modern gemmology, citrine's tawny colour was frequently confused with topaz, which is where trade names like citrine topaz came from. Topaz is 8 on the Mohs scale, a different mineral entirely.

Start from where these objects come from. The big decorative geodes are cut from basalt-hosted cavities in Brazil and Uruguay — the same source as the amethyst cathedrals, such as the 36 by 12 by 8 inch Brazilian amethyst cathedral that sold at Bonhams for US$2,040 including premium in the sale held 2–16 March 2019. A citrine cathedral of the same shape, from the same region, in the same basalt rind, is the same object with heat applied.

What to do about it:

• Assume treatment and ask anyway. Treatment disclosure is legally required in the United States, and a seller who will not put an answer in writing is telling you something.
• Expect the colour to hold. GIA describes heat treatments as durable and permanent under normal handling, though it notes intense heat can leave a stone slightly more brittle, which matters for thin crystal tips.
• Do not pay a natural-citrine premium. If a seller claims untreated natural citrine on a large geode, ask what evidence supports it, since GIA describes natural citrine as exceptionally scarce.
• Check the hardness, not the colour. Quartz at 7 will not be scratched by a steel nail. Yellow calcite, which is common in geodes, is 3 and a copper coin marks it easily.

Yes, in the sense that matters: it is real quartz, and its colour is stable. Heat does not add anything to the stone; it changes how the existing iron in the quartz behaves. GIA treats heat as one of the standard, durable gem treatments, and heated citrine has been the backbone of the yellow quartz market for decades.

Where it matters is price and honesty. Treated material should be sold as treated material, at treated-material prices, and United States rules require disclosure. A buyer who knows what they are buying and likes the colour has lost nothing.

The practical rule for a decorative geode: buy the object in front of you, for the colour and the shape you can see, and pay what that display piece is worth to you rather than what a rare untreated mineral would be worth.

• Ask in writing whether the piece has been heated. Expect yes on anything large and orange.
• Look at the rim, not the middle. Chips along the cut edge are the most common damage on cathedrals and the most expensive to hide.
• Check the base. A cathedral should stand square without a wedge under it.
• Weigh the freight. A piece over a metre tall is a shipping problem, and freight can approach the price of the geode.
• Keep the paperwork. A dated invoice naming the locality and the treatment is worth more later than any claim made verbally.

Photograph it and the app identifies it. Photograph the crystal tips in daylight, and a second frame of the outer rind, so the basalt shell and the crystal colour are both on record.