Quartz is often seen as crystal points, but can occur in other forms such as this egg shaped quartz.
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).
A variety of quartz, including amethyst, jasper, citrine, carnelian, agate, rose quartz, double terminated quartz, and rutilated quartz.
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.
Rutilated quartz occurs when quartz forms around rutile, resulting in a crystal that has little shoots of rutile running through it. Similarly tourmalinated quartz can form in the same way, though these shoots would appear black.
Quartz can also be rutilated, having golden rutile through it, or tourmalated which has black tourmaline in it. This occurs in quartz veins, where the quartz forms around the rutile or tourmaline. These minerals are essentiall 'trapped' inside the quartz.
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!
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
Specific Gravity: 2.65
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