Lemon quartz is a trade name for transparent quartz of a bright greenish-yellow hue, typically achieved through irradiation followed by a controlled heat treatment. It is a member of the macrocrystalline quartz family (SiO2), same species as amethyst, citrine and smoky quartz, and differs from those varieties only in colour and treatment route.

How the colour is made

Most material sold as lemon quartz starts as colourless quartz from Brazilian or African deposits. Exposure to gamma radiation - usually from a cobalt-60 source at a commercial irradiation facility - generates colour centres in the crystal lattice involving aluminium impurity ions and trace iron. A subsequent low-temperature anneal stabilises the colour and tunes the hue from the original smoky-greenish output toward a cleaner, brighter lemon yellow. The treatment is permanent under normal jewellery conditions but the colour can fade with prolonged exposure to direct sunlight or heat above approximately 200 degrees Celsius. Some commercial lemon material is sold under the name oro verde or green gold quartz, a related but typically slightly more greenish variant.

Naturally coloured greenish-yellow quartz of comparable hue does occur but is rare and not the source of commercial supply. Any stone offered as lemon quartz at trade-show pricing should be assumed to be irradiated, and the treatment disclosed.

Properties and identification

Standard quartz constants apply: refractive index 1.544-1.553, birefringence 0.009, specific gravity 2.65, Mohs hardness 7. Lemon quartz is uniaxial positive and shows no pleochroism strong enough to be useful for identification. Under longwave UV it is generally inert; the cobalt-60 history is not detectable by standard gemological tests, but the cleaner, more saturated greenish yellow distinguishes it from natural citrine, which tends toward a warmer, more orangey tone.

Cutting and trade

The material is plentiful and clean rough is inexpensive, so it is cut in larger calibrated and free-form sizes than most fine yellows. Large fancy cuts - cushions, ovals, briolettes and concave-faceted designer cuts - are produced for the silver and gold-jewellery market. It also serves as an affordable substitute or accent for citrine and yellow beryl. Pricing depends on size, clarity and cut quality, with stones above 10 carats commanding modest premiums where colour is even.

For the consumer, the relevant disclosure is that the colour is the result of irradiation; the treatment is stable under ordinary wear but the stone should be kept out of strong, prolonged sunlight, and steam or ultrasonic cleaning is generally safe but not necessary.