The latest issue of The Journal of Gemmology has an excellent article by Ulrich Henn and Rainer Schultz-Güttler called ‘Review of some current coloured quartz varieties’. For those who don’t have access to this journal, published by the Gemmological Association of Great Britain, this is a short summary to help you distinguish the different varieties.

Uncut amethyst crystal, prasiolite (11,5 ct), rock crystal (20,5 ct), amethyst (2,5 ct), citrine (natural?) (8 ct), citrine and rose quartz rough

We all know that quartz occurs naturally in various colours with varietal names – colourless rock crystal, yellow citrine, purple amethyst, pink rose quartz, brown smoky quartz, black morion, and the rare natural green prasiolite. Some of these colours can be produced artificially by radiation, sometimes coupled with heating. Artificial heating can also lighten some dark coloured quartz. Given the variations in natural quartz and their different reactions to irradiation and heating, a correspondingly wide range of artificial colours can be produced.

Most colourless natural quartz contains chemical impurities, even if these do not produce visible colour. They are mainly iron, in Fe-bearing quartz, and aluminium, in Al-bearing quartz. Natural or artificial irradiation with subsequent heating causes different colours in these two different quartz varieties.

First let’s consider Fe-bearing quartz. Naturally occurring clear Fe-bearing quartz crystals can experience low-level gamma irradiation from radioactive minerals in the surrounding rock. Over long periods of geological time this produces a change in the bonding of the iron impurity atoms, which results in the violet or purple colour of amethyst. Heating most amethyst to about 450°C causes it to bleach to colourless or pale yellow. Continued heating causes precipitation of iron oxide particles, which causes a deeper yellow. Most citrine on the gemstone market is such heat-treated amethyst. Because the colour is caused by uniformly dispersed iron oxide particles, this citrine is not pleochroic, that is, it does not show different intensities of colour when viewed in different crystallographic directions. Heat treated amethyst usually shows evidence of Brazil law twinning, often with colour zoning.

Some amethyst, when heated, turns green. Natural prasiolite is very rare and probably results from natural heating of amethyst. Most prasiolite on the gemstone market is artificially heated amethyst. This colour is produced by another change in the bonding of the impurity iron atoms, rather than by the precipitation of iron oxides. This material comes from only a few sources, mainly the Montezuma mine in Minas Gerais, Brazil.

Prasiolite itself can be subjected to artificial gamma irradiation and subsequent heat treatment to produce so-called ‘blueberry quartz’. This is a deep violet blue and resembles tanzanite.

Heating amethyst above 500°C not only bleaches it but can produce a milkiness, resulting in so-called ‘neon quartz’. This looks like lilac-coloured rose quartz. Stronger heating bleaches out the lilac colour completely and tiny water droplets form in the quartz. This material resembles adularescent gem materials and may be used as imitation moonstone.

Natural ametrine is bicolour quartz, purple and yellow, mainly from the Anahí mine in Bolivia. The colouring process is complicated. It involves differing concentrations of water in different growth sectors of the crystal and natural irradiation acting on the water to inhibit the formation of the purple colour in those sectors. Artificial heat treatment of ametrine to bleach the amethyst sectors can produce bicoloured citrine/colourless stones, sometimes marketed as ‘Lunasol’.

So much for Fe-bearing quartz. What about Al-bearing quartz? Usually the concentrations of aluminium in quartz are much higher than iron. Low levels of natural irradiation of Al-bearing quartz produces natural coloured citrine – yellow, yellow-green to yellow-orange. The details of the production of colour with irradiation in Al-bearing quartz are not well understood, but with increased levels of gamma irradiation the colour darkens, producing smoky quartz, and eventually black morion. Obviously, these colours also can be produced artificially with gamma irradiation, as is the case with the black Arkansas quartz.

Al-bearing quartz can also contain lithium in significant quantities. If is it lithium-poor, morion can be bleached by heat treatment to produce smoky quartz (and presumably some yellowish smoky quartz could be bleached to citrine). If Al-bearing morion, either naturally or artificially produced, is also lithium-rich, then gentle heat treatment at below 280°C can produce yellowish-green ‘lemon quartz’.

The distinctive characteristics of untreated and treated Al-bearing quartz are that if it is coloured it is pleochroic, and that it does not show evidence of Brazil law twinning. So, naturally coloured citrine will show pleochroism from pale to intense yellow and no Brazil law twinning, while citrine produced by heat treatment of Fe-bearing amethyst will show no pleochroism but Brazil law twinning may be present. Lemon quartz produced by heat treatment of Al-bearing smoky quartz will show yellow to yellow-green pleochroism but no Brazil law twinning.

Other quartz varieties exist too. ‘Greened amethyst’ is produced by artificial gamma irradiation of pale amethyst containing a very high content of water, from southern Brazil. The resulting crystals can be a deep green, with a ‘greasy’ lustre. This green quartz shows red under the Chelsea Colour Filter, while Fe-bearing prasiolite shows green. Heating above 500°C produces a cloudy opalescence due to exsolved water in very fine droplets.

Common pink rose quartz, generally described as ‘massive’ although it is crystalline, is thought to owe its pink colour to tiny included crystals of pink dumortierite. The cloudy appearance is due to scattering of light from these tiny inclusions. (In some material these inclusions must be crystallographically orientated, because they can produce asterism.) The much less common rosettes of pink quartz, usually on a white quartz crystal matrix, are essentially different from massive rose quartz. These clusters of pink single crystals have colour attributed to aluminium and phosphorus. Gamma irradiation can intensify the colour of these rosettes of single crystals to a stronger purplish pink.

The tendency of coloured quartz varieties to change colour on heating means that jewellers need to take care to avoid heating stones when repairing jewellery set with any coloured variety of quartz.  Duncan Miller