Citrine is yellow to orange quartz. Natural citrine — quartz that came out of the ground yellow — is genuinely uncommon, pale, and mostly from a few sources. The abundant, deeply coloured orange "citrine" in shops, in clusters and in tumbled stones is almost entirely amethyst (or smoky quartz) that has been heated until its purple turned yellow-orange. Both are quartz. Whether the difference matters depends on what the buyer was told.
What heating does
Amethyst owes its colour to iron in a particular oxidation state, produced by natural irradiation. Heating to around 400–500 °C alters that state; the purple disappears and a yellow-orange colour appears. Heating amethyst further, or heating certain material, gives green (prasiolite). The process is permanent and the product is stable.
Smoky quartz heated at lower temperatures also yields yellow and is sold as citrine.
Telling them apart
Colour. Heated amethyst tends toward orange, brownish orange and a distinctive "burnt" reddish orange, often with a colour concentration at the tips of crystals in clusters and a whitish or milky base. Natural citrine tends to be pale lemon to golden yellow, more even, and rarely as saturated. Colour alone is suggestive, not conclusive.
Colour zoning. Heated amethyst frequently retains the angular zoning of the original amethyst — now in orange rather than purple — sometimes with residual purple in the core of thick pieces. Natural citrine zoning is usually weak.
Pleochroism. Natural citrine shows weak pleochroism (two shades of yellow) under the dichroscope; heated amethyst shows none or almost none. This is one of the more useful bench distinctions, though it requires a clean, transparent stone.
Twinning. Heated amethyst retains the Brazil-law twinning of amethyst, visible as tiger stripes between crossed polarisers in the right orientation. Natural citrine is less consistently twinned. Supporting evidence.
Clusters and geodes. A cluster of orange crystals on a whitish base with the colour strongest at the tips is heated amethyst, effectively without exception — natural citrine clusters of that appearance do not occur.
Spectroscopy. Where the distinction matters commercially, infrared and UV-Vis spectroscopy can separate the iron-related colour mechanisms.
Does it matter?
For most buyers of tumbled stones, points and clusters, the distinction is academic — both are quartz, both are the colour they see, both are inexpensive. It matters in three cases: when a stone is sold as natural, untreated citrine at a premium; when a tradition or practitioner specifies untreated material; and when the buyer simply wants to know. A laboratory can usually reach an opinion from the features above and should state it as such.
What else is sold as citrine
Yellow glass. Singly refractive, bubbles. Quartz vs glass.
Synthetic citrine. Hydrothermal yellow quartz exists and is identified as synthetic amethyst is — by growth zoning and twinning. Amethyst: natural, synthetic and glass.
Yellow topaz, yellow beryl, yellow sapphire. All heavier and higher in RI; sometimes citrine is sold as these (particularly "topaz"), which is the reverse problem.
Frequently asked questions
Is heated amethyst "real" citrine?
It is yellow quartz, which is what citrine means mineralogically. It is not natural citrine, and if sold as natural it is mis-described. Many in the trade simply call it citrine; a careful seller says "heat-treated".
Is heated citrine stable?
Yes. The colour does not fade under normal conditions.
How can I tell at home?
Orange, burnt-looking colour concentrated at crystal tips on a white base almost always means heated amethyst. Pale, even lemon yellow is more likely natural. Beyond that, a laboratory is needed.
Is natural citrine more valuable?
Modestly, in collector terms, because it is scarcer. Commercial prices for both are low.
Is "Madeira citrine" natural?
The name describes a deep orange-red colour and is applied almost exclusively to heated material.