"Rock of the Month #206, posted for August 2018" ---

QUARTZ is the most abundant mineral in the Earth's crust. It is a major (in a few cases the only) constituent of diverse granitoid igneous rocks, sandstones and quartzites, as well as desert, river and beach sands. Related material occurs also in cryptocrystalline and fibrous forms as chert (flint) and in a gel-like state as opal. There are many varietal names. Pure quartz is clear and colourless, known as "rock crystal". The hexagonal prismatic crystals are well-known. Although it is composed almost entirely of silica (SiO₂) a small proportion (much less than 1%) of substitution by minor and trace elements such as iron can control the colour. The nature of quartz is to be allochromatic, meaning that it can occur in a wide range of colours, from clear and colourless to virtually black. This variability is shared by a relatively few discrete mineral species, such as fluorite and sphalerite, smithsonite and calcite. Colour variability is seen also in mineral families, such as garnets and tourmalines, though here there is more than one "end-member" mineral species involved, and so major-element composition changes are also present.

Quartz is hard (Mohs hardness 7), tends to be clear and translucent, except when filled by mineral or fluid inclusions, and is quite tough, such that it can be carved, faceted and polished, and makes nice durable jewellery which is inexpensive compared to, e.g., the "big three" coloured stones, ruby, sapphire and emerald. Good-quality quartz, then, is perhaps the quintessential semi-precious stone. Quartz and its varieties are summarized by Thomas (2008, pp.92-98).

Mineral identification

So, what have we here? The range of colour brought to mind natural quartz, though some examples are more convincing than others. As a mineralogist, I hesitate to "call" the identities of many gemstones. In part this is a matter of experience, but also of habit: I tend to see minerals in the rough, and often identify them in thin slices under the microscope, where optical properties tend to follow the variation spelled out in textbooks and reference works. Gemstones, like other hand specimens, vary in size and shape, and some properties, such as colour, tend to be more intense as the thickness of the specimen increases. A gemologist has to take account of such factors and, of course, is generally limited to non-destructive methods of testing! As a further complication, the stones are often crafted within artful rings and other mounts, which further constrain the tests that can be conducted without disassembling the jewellery.

After naming the minerals suggested by colour (e.g., maybe garnet or topaz for the two orange-brown examples in the bottom tier), I also considered the varietal names of quartz, and decided that all might in fact be quartz (see figure caption), or even glass, and that these nine pieces might be some kind of jeweller's display set to teach elements of faceting. The nine stones vary from 22 to 49 carats (1 carat = 200 milligrams), total 267 carats.

1. Density. I decided to limit the choice by the one safe test at my disposal, to weigh each piece in air and water, and accurately determine the density (specific gravity) of each piece. Density and S.G. are closely related, but not identical ..... In brief, density is the mass of a material per unit volume, thus D=M/V, expressed in units such as grams per cubic centimetre. S.G. is similar but not identical, and is more reproducible in that the conditions of measurement are taken into account, and is measured by comparison with a reference material. The latter is typically water at its densest (+4°C) and 1 atmosphere pressure. S.G. of a solid is determined by weighing the sample in air and in water, and noting the temperature. For a detailed discussion see this Wikipedia article.

The S.G. of quartz is 2.65, and I use two samples (one a milky quartz, a typical vein quartz, used in this test) and a clear "rock crystal" prism, as standards. I made the measurements, and eight samples came out at an average of 2.65±0.02 (2 s.d.), the standard at 2.64, consistent with quartz. I did not do a full re-check, drying in an oven at low heat, as I do for the most critical tests, but I later re-measured the ninth sample and the standard. This last sample, the purple stone, came out at 2.75 each time, slightly (3.7%) but significantly denser than the rest (data file: see the relevant S.G. spreadsheet). Despite the "amethyst" result, the data reinforced my suspicion that all could be varieties of quartz, or maybe glasses of S.G. matched closely to quartz but, what other tests could we try?

2. Optical properties. Gemmologists work with stones in a wide size range. The ways in which stones interact with light offer many clues to their identity, since each mineral species has a particular crystal structure. A basic optical characteristic is refractive index, which is related to the speed with which light travels through a medium. Depending on the class of crystal involved, minerals have one, two or three refractive indices, which can be measured in a properly oriented crystal.

Seeking some advice, I made some enquiries, and took the samples to the 55th Annual Gemboree in Bancroft, Ontario, on 03 August. Here I was directed to the desk of Karen E. Fox, MSc, FGA, FCGmA, applied physicist and gemmologist, who was operating a table of specialist equipment for gem identification. Though time was of the essence, she managed to examine each of the nine stones. At least eight of them returned the "bull's-eye" optical pattern indicative of a uniaxial material such as quartz (the hardest to see was the especially large, thick, dark brown sample seen at lower right in the photo). The right combination of fibre-optic lighting, polarizing filters, magnification, plus skill and experience revealed not only that the suite are all quartz, but that at least five are synthetic, grown little by little from flat seed plates, and so examples of the synthetic gemstones that have a long history, starting of course with attempts at recreating the most expensive stones, such as ruby and diamond.

It appears that, at a minimum, stones 1 (zoned), 3 (amethyst), 4 and 8 (green), and 9 dark (zoned orangey-brown) are all synthetic. This accords with the fact that the colours of 1,4 and 8, at least, are atypical for quartz. The rose quartz, however, shows signs of Brazil-law twinning, which suggests a natural quartz. However, it appears clear, and layered with a colourless base and distinctly pink crown, so it may actually be a synthetic too, and could just as easily be termed a pale amethyst.

To my own aesthetic sense, the synthetic origin of the majority of the suite detracts but little from their interest and beauty. How this would affect the appraisal of the collection is quite beyond my field of expertise. We might add that, in the gem world, there are issues of heat treatment, irradiation and other methods for "improving" on Mother Nature that also affect the appearance of stones, and further complicate the arcane art of gemstone pricing. The pricing catalogue of Miller and Sinkankas (1994), though dated now, indicates just how complex things can get, with many prices for each of 22 categories of quartz and chalcedony (ibid., pp.174-181).

Colour and other qualities, I

As mentioned above, a small amount of an impurity can influence the colour of a mineral. Impurities can be dissolved in the crystal structure ("solid solution") or can occur as small inclusions of other minerals. In the case of quartz, the latter include well-known attractive forms with elongate prisms of rutile (TiO₂) or of the tourmaline family of borosilicates. Another attractive variation is clear quartz (rock crystal) flecked with large bubbles ("fluid inclusions") that are variably filled with liquid (aqueous solutions), gas and sometimes mineral inclusions (such as flakes of graphite). A common type from New York State displays all these bubbly features, and these crystals, typically attractive, colourless, stubby prisms, are called "Herkimer diamonds".

The topic of mineral colour is surprisingly complicated. Some of the colour variation in quartz has been explained by factors, acting alone or together, such as:

• Small amounts of Fe, Al, Ti, Ni, OH (hydroxyl) and hydrocarbons.
• Tiny mineral inclusions trapped within the quartz host.
• Radiation damage by proximity to U and Th-rich minerals.
• Defects in the crystal structure ("colour centres"), associated with the trace elements such as Al.

Synthetic quartz

Synthetic gemstones, their origins, and the fakes known as schlenters are described concisely by Thomas (2008, pp.198-207). Electron microscopy revealed that synthetic quartz develops fibrous and ribbon structures prior to the full quartz framework structure (Arnold and Guillou, 1983), and that impurities such as Al and hydrocarbons may be significant in nucleation and crystal growth. Synthetic quartz has been made for many years by hydrothermal synthesis (Liddicoat, 1977, pp.110,148): as of the time of writing (the 1970s), it had not been much used for jewellery, but coloured forms were eventually made in both Russia and the USA, citrine and amethyst, and also yellow-brown, blue and green hues not seen in nature. Some synthetic quartz can be identified by the colour banding parallel to the flat "seed plate" from which a nucleating crystal propagates. Millions of carats of synthetic citrine were shipped from the USA to Japan, so this would surely appear on the market. Synthetic quartz is often recognized by banding parallel to the seed plate, and by irregular "bread-crumb" inclusions, most abundant close to the plate (Liddicoat, 1977, p.148). Two decades later, Lurie (1999) reviewed the role of synthetic gemstones in the gem business. One issue was the problem of synthetic quartz (especially amethyst and citrine) being sold as natural quartz, a problem dating to the early 1990s. Modern synthetic amethyst (mostly from China and Russia) is harder to spot than earlier forms, which often had the "bread crumb" inclusions: the modern material emulates Brazil-law twinning, and so looks very natural, and is especially prevalent in 0.5 to 2-carat sizes. One dealer (as of 1999) had stopped purchasing cut amethyst, and instead bought rough from Zambia and Bolivia.

Colour and other qualities, II

As noted above, the many colour varieties of quartz are summarized by Thomas (2008, pp.92-98). Unlike some colours, rose quartz is a common variety, and cherished for its delicate hue. Goreva et al. (2001) extracted pink nanofibres from rose quartz from 29 pegmatite and massive vein localities in Brazil, Nevada, Montana and elsewhere. These nanofibres are only 0.1 to 0.5 microns wide (for comparison, a hair on your head is circa 30 microns, 0.03 mm, thick!). The fibres may be a phase akin to the pretty blue (to pink) borosilicate, dumortierite, but the detailed explanation is more complex than this. Cohen (1989) determined that the colour of smoky quartz is related to Al⁺³ impurity, while iron is responsible for the mauve colour of amethyst. Iron occurs in channels parallel to the c-axis and the a-axes of the quartz structure, and in channels normal to the major rhombohedral faces. The Fe in the channels produces Brazil twinning to relieve strain in the structure, and thus Brazil twinning is normal in material with the amethyst hue. Yellow Brazil-twinned quartz will acquire the amethyst colour when subjected to ionizing radiation, in nature or in the laboratory, which explains the occurrence of so-called ametrine (a combination of amethyst and citrine) in localities such as Santa Cruz, Bolivia. Radiation damage develops around radioactive atoms within the range of alpha particles in quartz, some 35-45 microns (Botis et al., 2005). Iron is no doubt involved in other quartz, jasper and quartzite varieties such as yellow sard and orange carnelian. Larger haloes in mica had been explained in terms of radiation damage around postulated extinct, superheavy elements. However, damage beyond the range of the most energetic common alpha particles in mica (39 microns), with radii out to 105 microns, may also be explicable in terms of colour centres in micas and in quartz surrounding monazite grains. The smoky quartz has a grey colour induced by ionizing radiation, explicable in terms of an Al -based colour centre (the Al substitutes for a little of the Si in quartz: Odom and Rink,1989). Blue quartz may be generated by virtue of minute inclusions of ilmenite (FeTiO₃). Green quartz is unusual, but not unknown. Hebert and Rossman (2008) describe green quartz from the Thunder Bay amethyst district, northwest Ontario. Green quartz also occurs in Bahia (Brazil), Zambia and Namibia. Hydroxyl (OH) groups appear to be related to the green colour though other causes are possible (e.g., nickel content in bright green chrysoprase).

Footnote

The optical crystallography textbooks teach you how to orient a crystal parallel to its crystal axes, something of an art (e.g., Kerr, 1959; Bloss, 1961; Nesse, 2004; Dyar et al., 2008). Though the concepts take some careful reading, the optics of common minerals such as quartz and calcite can take you from Herkimer diamonds to trilobite eyes to the navigational prowess of the Vikings, as explained in an enthusiastic technical essay by Skalwold and Bassett (2015).

Acknowledgements

Brief but valuable discussions with Mary Garland and (at the Gemboree) Brad Wilson, Bob Beckett and especially Karen Fox are all much appreciated. No matter how long you study rocks and minerals, there is always more to learn!

References

Arnold,M and Guillou,J-J (1983) Croissance naturelle de paracristaux de quartz dans une saumure sulfatée calcique à basse temperature. Bull.Min. 106, 417-442 (in Fr.).

Bloss,FD (1961) An Introduction to the Methods of Optical Crystallography. Holt, Rinehart and Winston, New York, 294pp.

Botis,S, Nokhrin,SM, Pan,Y, Xu,Y, Bonli,T and Sopuck,V (2005) Natural radiation-induced damage in quartz. I. Correlations between cathodoluminence colors and paramagnetic defects. Can.Mineral. 43, 1565-1580.

Cohen,AJ (1989) New data on the cause of smoky and amethystine color in quartz. Mineral.Record 20 no.5, 365-367.

Dyar,MD, Gunter,ME and Tasa,D (2008) Mineralogy and Optical Mineralogy. Min.Soc.Amer., 708pp. plus DVD-ROM.

Goreva,JS, Ma,C and Rossman,GR (2001) Fibrous nanoinclusions in massive rose quartz: the origin of rose coloration. Amer.Mineral. 86, 466-472.

Hebert,LB and Rossman,GR (2008) Greenish quartz from the Thunder Bay Amethyst Mine Panorama, Thunder Bay, Ontario, Canada. Can.Mineral. 46, 111-124.

Kerr,PF (1959) Optical Mineralogy. McGraw-Hill Book Company, New York, 3rd edition, 442pp.

Liddicoat,RT (1977) Handbook of Gem Identification. Gemmological Institute of America, 10th edition, 440pp.

Lurie,M (1999) Dealers push for synthetics cleanup. Colored Stone 12 no.3, 123,142-144, May.

Miller,AM and Sinkankas,J (1994) Standard Catalog of Gem Values. Geoscience Press, Inc., Tucson, 2nd edition, 271pp.

Nesse,WD (2004) Introduction to Optical Mineralogy. Oxford University Press, 348pp.

Odom,AL and Rink,WJ (1989) Giant radiation-induced colour halos in quartz: solution to a riddle. Science 246, 107-109, 06 October.

Skalwold,EA and Bassett,WA (2015) Double trouble: navigating birefringence. Min.Soc.Amer. booklet, 18pp.

Thomas,A (2008) Gemstones: Properties, Identification and Use. New Holland Publishers (UK) Ltd, 256pp.