Quartz

The most common mineral on the face of the Earth is quartz. It forms all over the world in a variety of environments and is a component in most rocks. It forms in igneous environments in granites and pegmatites, hydrothermal veins, as an accessory in most ore bodies, sea deposits as flint/chert, sedimentary deposits of sandstone, and is metamorphosed into quartzite. For sheer variety, quartz beats all other minerals hands down.

Some common varieties of quartz are:

• Crystalline Quartz - These are varieities of quartz that form large, well-formed crystals.
  • Rock Crystal Quartz - Colorless quartz, pure SiO₂
  • Amethyst Quartz - The purple color of amethyst is due to the presence of iron as [FeO₄]^4-. Amethyst is commonly found in areas of igneous rocks that are high in iron. As water containing dissolved quartz seeps through cavities in basalts, it picks up some of the iron conatined in these rocks. The quartz is then precipitated (as well as the iron) yielding amethyst quartz.
  • Rose Quartz - Rose quartz is pink and is colored by manganese (Mn). It is usually highly fractured and cloudy and so is rarely used as a gemstone.
  • Smoky Quartz - Smoky quartz is gray/brown in color due to Al³⁺ ions that produce this color when colorless quartz is exposed to radiation.
  • Citrine Quartz - Citrines yellow color resembes certain varieites of topaz. This leads to confusion between the two minerals and citrine is sometimes referred to as yellow topaz.
  • Milky Quartz - The cloudy white color of milky quartz is due to many tiny air bubble inclusions.
• Cryptocrystalline Quartz - These are varieties of quartz that form crytals to small to be seen even with magnification.
  • Chalcedony - Chalcedony is the term applied to most cryptocrystalline quartz. It comes in a variety of colors and can also be colored artificially. Deposits of chalcedony form from solution and are often found lining cavities and crevices in caves.
    • Agate - Arguably the most recognizable type of microcrystalline quartz, agate is chalcedony that often forms as multi-colored bands, though sometimes irregularly distributed or in fernlike patterns.
    • Onyx - Multi-colored chalcedony layered in parallel bands.
    • Heliotrope (Bloodstone) - A green chalcedony with red spots.
    • Chrysoprase - An apple-green chalcedony colored by nickel-oxide.
    • Carnelian - A red chalcedony colored by iron.
  • Jasper - A granular microcrystalline variety of quartz colored red because of hematite (iron) inclusions.
  • Flint/Chert - These are made of tiny crystals of quartz. Darker nodules are called flint, whereas lighter beds are called chert. Both varieties were a common material for making tools in early human societies.
• Other Quartz Varieties
  • Tiger's Eye - Yellow/brown or blue fibrous material that forms by the replacement of riebeckite by quartz. The brown color comes from oxidized iron mixed in with the quartz. Without the iron, the mineral retains the blue color of riebeckite.
  • Rutilated/Tourmalated Quartz - Crystalline quartz enclosing needles of rutile or tourmaline. When the quartz is clear it is commonly cut as a gem.

Specimens: