Stone of the Week - Quartz

For the first in my series about stones, I figured Quartz is a good place to start since there is such a variety of it. Quartz is the most abundant mineral on the Earth's surface, due to its resistance to weathering. When it does weather, it is still often in pieces or grains that end up in igneous, metamorphic, and sedimentary rocks. Quartz is a versatile stone, used for many things besides jewelry, such as lab equipment and industry. The ancient Romans cut quartz into round balls that were polished and carried around by wealth Romans to cool their hands during the summer months. One weighting 106 pounds is currently on display at the National Museum in Washington D.C.

Quartz is often seen as crystal points, but can occur in other forms such as this egg shaped quartz.

Quartz comes in many forms, usually as hexagonal prisms, although these are often imperfect. It can also form as large massive bodies. Quartz crystals can grow to enormous sizes – one in Brazil weighed 40 tons, while another in Kazakhstan was 70 tons. Quartz crystals can be singularly terminated (has one point) or as a double terminated crystal (two points on either end) which is less common. It is also common in geodes, which are round rock formed by bands of minerals (often some type of quartz) accumulating on top on each other, and can have a void in the center with crystals.

Examples of Quartz inside geodes

The variety of color in quartz is due to impurities within the crystal, and a number of these have different names. Chalcedony is a cryptocrystalline (microcrystalline) form of quartz with very small crystals – hence you don’t get the hexagonal crystals but often appears as smooth and banded. Other cryptocrystalline quartz includes agate (multicolor, banded), onyx (like agate with straight bands, multi color or black), carnelian (reddish orange), jasper (opaque, multi-color), aventurine (often with shimmering micas, commonly green), prase (green), plasma (dark green), heliotrope or bloodstone (like plasma but with dark red spots), sard (reddish-brown), and moss agate (has dendritic patterns of manganese).

Other quartz varieties include amethyst, which is a purple color due to the presence of manganese or ferric iron, and is perhaps the most valuable form of quartz as a gemstone. Rose quartz gets its color from traces of manganese or titanium, and can lose its pink color when heated. Smoky quartz is a dark brown color, is caused by irradiation; a very dark brown /black type is also known as morion. Citrine is commonly a pretty yellow or orange color due to inclusions of iron hydrates. Tiger’s eye is a fibrous form of quartz that is typically a yellow-brown color, but with the addition of impurities can appear as other colors.

A variety of quartz, including amethyst, jasper, citrine, carnelian, agate, rose quartz, double terminated quartz, and rutilated quartz.

Quartz can also be rutilated, having golden rutile through it, as seen in the photo above (tumbled yellow/white piece) or tourmalated which has black tourmaline in it. This occurs in quartz veins, where the quartz forms around the rutile or tourmaline.

A number of these varieties will be featured later for a more in depth look. There will also be a review of terminology used in the 'stone facts' and how to test stones. Amethyst will be the next in my series, so check back to learn more about this beautiful purple gemstone.


If you have a stone you would like to know about, please feel free to leave a request in the comments section. Have a mystery stone? Leave a link in the comments to a picture of it, and it may be featured as a part of this series!



Quartz Facts:
Chemical composition: SiO2
Color: Most common white/clear, but occurs in many colors
Habit: Commonly 6 sided (hexagonal) prisms, also massive
Fracture: Uneven and conchoidal
Cleavage: None
Luster: Vitreous
Hardness: 7
Specific Gravity: 2.65
Streak: White
Occurrence: Worldwide


References
A Guide to Rocks and Fossils by B. Busbey III, R. R. Coenraads, P. Willis, and D. Roots. Published 2002 by Fog City Press. ISBN: 1877019518

Rocks, Minerals, & Fossils of the World by C. Pellant and R. Phillips. Published 1990 by Little, Brown and Co. ISBN: 0316697966

Wikipedia – Quartz. http://en.wikipedia.org/wiki/Quartz