Citrine

A yellow to yellow-orange or yellow-green variety of quartz.
NOTE: Quartz colored by visible inclusions, or coatings, of any kind is not called citrine. Iron-stained quartz should not be mistaken for citrine.. Microscopic to Nano-scale inclusions may be a legitimate cause of colour for citrine.



The cause of the color is still under debate. Like smoky quartz, natural citrines pale when heated above 200-500 °C and turn yellow again when irradiated (Lehmann, 1970), and they also slowly pale in UV light. This indicates the presence of color centers. At least some citrine is colored by aluminum-based and irradiation-induced color centers related to those found in smoky quartz (Lehmann, 1972; Maschmeyer et al., 1980). Accordingly, transitions to smoky quartz ("smoky citrine") exist, many citrines show smoky phantoms. There appear to be at least two types of yellow Al-based color centers with different thermal stability (Schmetzer, 1988). Since the yellow color centers are often more stable than the smoky color centers, some smoky quartz can be turned into citrine by careful heating (Nassau and Prescott, 1977). Natural citrine, as well as citrine produced by heating smoky quartz, is dichroic in polarized light.

It has also been suggested that iron is the cause of color, as artificial crystals grown in an iron-bearing solution turn out orange. However, the dichroic behavior of the lab-grown crystals differs from that in natural citrine (Rossmann, 1994), and the UV-sensitivity of natural citrines indicates that the color is not simply caused by incorporated trace elements.

Citrine can be found in the same geological environments as smoky quartz and is often associated and intergrown with it. Contrary to amethyst, citrine crystals may show a macromosaic structure and become very large (meter-size crystals of good quality have been found Minas Gerais, Brazil).

Note: Natural citrine is very rare. Large quantities of amethyst, usually of lesser quality, are heated to turn it yellow or orange and sold as "citrine." Because the color is now caused by finely distributed iron minerals (mostly hematite and goethite), heated amethyst is not citrine in the strict sense, and also shows no dichroism in polarized light.
That a crystal shows dichroism does not mean it is a natural citrine, it just means it is not heated amethyst. Certain smoky quartz and rock crystals can be turned yellow by careful heat treatment and/or irradiation, and these crystals will show dichroism.

Thin coatings of iron oxides on colourless quartz, as well as inclusions of yellow iron oxides ("limonite"), may simulate citrine.