Rose Quartz

 

last modified: Tuesday, 26-May-2009 03:45:00 CEST

Document status: complete


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Rose  quartz is a pink variety of quartz that occurs in large translucent masses. It is never transparent, and it does not form crystal faces or crystals. It is not a cryptocrystalline variety, however, as it is made up of many intergrown crystal subindividuals. Rose quartz can be grainy, but the material used for lapidary purposes is built more homogeneously, like the specimen of unknown origin to the right.

The pink variety of quartz that forms crystals is called pink quartz and is covered in its own section. Despite being macrocrystalline varieties of similar color, rose quartz and pink quartz should be treated as different varieties as:

- the cause of their color is not the same,
- pink quartz is sensitive to light, while rose quartz is not
- both varieties form in different environments, and
- pink quartz develops crystals while rose quartz does not.

Rose quartz is used as an ornamental stone or as decoration and is one of the most popular semiprecious gem stones.

 

Specific Properties

Rose quartz never forms crystals. Quartz crystals of a similar color are called pink quartz.

The color can vary from a very bright pink to almost purple. Rose quartz is usually very evenly colored.

The debate about the cause of the color is slowly settling, although there are still a few open questions. For the past 30-40 years it was assumed that rutile TiO2 inclusions are the cause. Other proposals included the presence of irradiation induced color centers, similar to amethyst or smoky quartz, either in included minerals, or in the quartz substance itself. The color indeed disappears upon heating rose quartz, but at temperatures above 500°C, much higher than the temperatures required to bleach amethyst, certain citrines and smoky quartz, which do have irradiation induced color centers. Heated rose quartz will not regain its color upon irradiation. Irradiation (both natural or artificial) of rose quartz can lead to the formation of additional smoky quartz centers and according alterations in color. It is therefore unlikely that the color is caused by irradiation induced color centers. On the other hand there are reports of rose quartz from certain locations (Brazil) fading when exposed to sunlight, which supports the presence of color centers.

More recently it has been suggested by Applin and Hicks (1987) that fibrous inclusions of the mineral dumortierite (Al,Fe)7 [O3 | BO3 | (SiO4)3] are responsible for the haze and color of rose quartz. This first sounds like an odd suggestion, as dumortierite is often deeply blue (even the color of its streak), but there are pink variants.

The nature of the colorizing agent in rose quartz has recently been studied by Goreva, Ma and Rossman (2001), following the more "direct" approach that was already chosen by Applin and Hicks: they dissolved rose quartz from various locations in hydrofluoric acid and extracted mats of a fibrous, rose-colored mineral that is related, but apparently not identical to dumortierite. That mineral makes up only about 0.05% - 0.15% of the overall weight of rose quartz. The mineral will bleach when heated above 500°C, but will not regain its color when irradiated. Color loss can be induced by oxidizing agents, too, and heat treatment in a reducing atmosphere will turn heated and paled material rose-colored again.


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Star rose quartz  shows asterism when illuminated by a point-like light source: light reflections appear as a six-rayed star. It can best be observed in a polished ball, and is invisible under diffuse light. Star rose quartz is only found at a few locations, the one in the image is from pegmatites in the Vorondolo Mountains, south-east of Antsirabé in Madagascar. The star is caused by light reflected from needle-like inclusions of the aforementioned mineral that is also the colorizing agent. These needles grew oriented along three axes that intersect at an angle of 60°. The star's position depends both on the location of the light source and the position of the observer and is different from the surface reflection because refraction acts on the light rays that enter and leave the sphere. Because of that the left and right eye see the star at different positions and the brain concludes from this stereo image that it must hover above the rose quartz ball - looking at it for some time can make you feel dizzy as something is apparently "wrong". The perceived position of the star and its relation to the orientation of embedded fibers has been studied by Killingback, 2005, Schmetzer and Krzemnicki, 2006, and Wüthrich and Weibel, 1981. A similar star can be seen in other minerals with regular fibrous inclusions, too, star sapphires and rubies, for example, are well known for this phenomenon, and belong to the most valuable gemstones.

Although the color is caused by embedded mineral fibers, rose quartz is dichroic. The color change from a deep to a pale rose color is easily observed using a polarizer lens. This is very remarkable and is an indication that the embedded mineral fibers are not randomly oriented, but preferrably oriented along specific crystallographic axes. This is also supported by observations on star rose quartz: when viewed along the "poles" of the sphere, defined by the two spots on the sphere where the surface reflection and the star's center coincide, there's no dichroism (the color does not change upon rotating the polarizer lens). When viewed along the "equator" the dichroism is bold, and when the polarizer lens is kept fixed and the sphere is rotated around the poles, the color does not change either, indicating that the preferred polarization plane remains the same.

 

Occurrence

Rose quartz occurs in pegmatites. It forms at very high temperatures, between 400°C and 700°C, thus sometimes beyond the inversion temperature at which the transition from low to high quartz takes place (573°C). These conditions could be called pneumatolytic, that is, the rose quartz grew in a gaseous supercritical phase, not from a watery solution, as most other quartz varieties do. Rose quartz is also found in massive hydrothermal veins, but more rarely.

The fact that the fibers are of submicroscopic size and are at least in certain cases embedded in the quartz substance parallel to certain crystallographic axes is a strong indication that these are epigenetic inclusions that formed by exsolution of trace elements from the crystal lattice: the high temperatures during formation allowed the incorporation of certain small ions into the lattice. When temperatures were falling and the lattice contracted, the trace elements were slowly "squeezed" out of the quartz lattice and formed fibrous inclusions. So far this is speculation, however.

Rose quartz is usually mined manually from pegmatites, explosives would shatter the rocks and cause fine cracks in the specimen that decrease their value.


 

Locations and Specimen

Rose quartz occurs at many pegmatite locations all over the world. It is commercially mined in Madagascar, South-Africa, Namibia, U.S.A. (South Dakota), and Brazil (Bahia and Minas Gerais). Star rose quartz is found in South Dakota and Madagascar.


Madagascar


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Rose  quartz from Ambatondrazaka in southern Madagascar has grown around a long and deeply colored aquamarine crystal, a blue-green beryl typically found in pegmatite environments. Parts of the rose quartz are smoky.


Namibia


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The  most interesting thing about this rose quartz is that I know its origin: It is from the Rössing Mountains near Swakopmund. Even close visual inspection does not reveal any structural difference from the specimen in the first picture on top of this page, just the color is a bit more pale and bluish.


 

Further Information, Literature, Links

The aforementioned paper by ->Goreva et al. discusses the nature of rose quartz coloration and is available online.

Full size scanning electron microscope images of the fibers can be found at http://minerals.gps.caltech.edu/Manuscripts/2001/RoseQuartz1/Index.htm, a web page of George Rossman's mineralogy research group at Caltech. A more detailed description of the pink mineral is found at http://minerals.gps.caltech.edu/Manuscripts/2002/RoseQuartz2/Index.htm.

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