Quartz (SiO2) is a common mineral found in all three classes of rocks (igneous, metamorphic and sedimentary), in many environments and in a range of colours. However, rose and blue quartz are less common than some of the other varieties. This article discussed these two extraordinary minerals.
Rose quartz has a pale pink to rose-red colour, thought to be caused by trace amounts of titanium, which absorbs all colours except pink. In a laboratory experiment, samples of rose quartz from several localities were carefully dissolved in acid. The remaining insoluble residue consisted of thin microscopic fibres, which may also be responsible for the colour of rose quartz.
Well-formed rose quartz crystals are rarely found in nature, but when they are, they are generally found in massive chunks associated with pegmatites (Fig. 1). The term pegmatite refers to exceptionally coarse-grained crystalline granite. Since rose quartz is cloudy, it is not popular as a faceted gem, but it is commonly made into cabochons (Fig. 2), rounded into beads for necklaces or carved into various objects.
It has been named as South Dakota’s official state mineral. Here, rock hounds have a good chance of finding specimens ranging from shades of light pink to rose-red. Some rose quartzes from South Dakota also have a distinctive asterism, that is, a star-shaped display of light on the polished surface.
Blue quartz, with a deep to sky blue colour, is packed with tiny grains of other minerals, such as rutile (TiO2) and ilmenite (FeTiO3). Other inclusions include tourmaline, crocidolite, magnesioriebeckite, zoisite and several others. Some researchers hypothesize that the blue colour comes from the ‘Rayleigh scattering’ of light by these microscopic inclusions. Rayleigh scattering selectively scatters visible light of the shorter blue wavelength (for example, to give the sky its blue colour). However, the reason for the blue colour still remains uncertain.
Blue quartz has a waxy lustre and sometimes displays asterism. It occurs at a number of localities, including in Llano County, Texas, where it is found as small, doubly-terminated crystals in a rhyolitic porphyry, informally called llanoite. The crystals weather out of the host rock and can easily be collected. Blue quartz is also found in a diorite near the Dairyland Power Dam near Tony, Wisconsin. It was also recently discovered in the Cushing Point Formation at Peak’s Island, Maine. The specimens there have inclusions with the chemistry of biotite. In the past, this mineral has not been listed as a possible inclusion. However, research now suggests that the inclusion of biotite on Peak’s Island may be responsible for giving quartz its blue colour.
Blue quartz is also associated with pegmatites of the Cape Ann Granitite at Andrew’s Point in Rockport, Massachusetts (Figs. 3 and 4). In addition, I have found blue quartz at two Colorado locations: Park County, near Hartsel and on the tailings of the Bull Domingo Mine in Custer County, northeast of Silvercliff. A famous site – Antequera, near Malaga, Spain – yields translucent crystals of intensely blue quartz.
While some varieties of quartz are well known (such as amethyst and smoky quartz), blue quartz is a lesser known variety. It is wonderful to behold and exciting to find in the field. The rich blue colours hold your attention and move you to plan a collecting trip. And, the variable rose colours of rose quartz beckon the collector to cut and polish rough slabs of this mineral. Both varieties truly deserve a spot in your collection.
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