As the name suggests, "smoky quartz" is a type of macrocrystalline (meaning it occurs naturally in large, visible crystals) quartz that has a smoky appearance. Some varieties appear brown or grey, or so-called "coon tail" (with a banded appearance) and while it almost always appears somewhat translucent when found in nature, it can be so "smoky" as to be completely opaque. The cause of the smokiness is not entirely understood, but it's well-accepted that long-term exposure to radiation generated by surrounding isotope-bearing rocks (particularly granite) is the key factor in the coloration that results. The most commonly accepted theory is that long-term exposure to radiation alters the oxidation states of natural impurities found in the quartz, which results in dark coloration.
Smoky quartz can be artificially clarified into "clear quartz" by exposing it to intense heat. Conversely, "clear quartz" can artificially be "smokified" by exposing it to intense radiation. Those who buy quartz at rock shops must be wary of the fact that smoky quartz is often artificially treated to make it appear darker, which diminishes the whole experience of obtaining a natural, beautiful mineral obtained from the depths of the Earth. Generally, the darker and more opaque it is, the more likely it was to have been altered by artificial sources of radiation.
Smoky quartz often contains inclusions of carbon dioxide and water, which, if abundant enough, can give it a white appearance, and it is then referred to as "milky quartz." Many pieces are often a combination of "smoky" and "milky" quartz. Occasionally, smoky quartz contains "phantoms," which appear to be little crystal terminations embedded within the crystal itself, which represent the crystal at a different stages of its existence. As it grows larger, the phantoms are enveloped by the crystal and appear stuck in time, a visual reminder of the smaller crystal that once was. I have in my hands here an awesome three inch piece with phantoms composed of amethyst that start halfway up the crystal and continue to the very tip. Many other minerals occur as natural inclusions in smoky quartz, including but not limited to rutile (a source of titanium), tourmaline, hematite, and goethite. There exists a particularly cool blend of seven minerals (often referred to as the "super seven" stone) including smoky quartz, amethyst, clear quartz, rutile, cacoxenite, goethite, and lepidocrosite. The other day when I actually purchased the aforemention three-inch specimen, the proprietor of the rock shop handed me a beautiful chunk of super seven absolutely free, and I grew instantly giddy in anticipation without having known before what the hell it was. He pointed it out to me in a book, and I was most grateful for his generosity.
Smoky quartz has a hardness of seven, no cleavage, fractures conchoidally, and has a vitreous luster. Smoky quartz is commonly found in Colorado, Brazil, and Arizona, among other places. To those who are interested in the possible metaphysical properties of smoky quartz, it is said to transform negative energies and emotions such as anger, stress, and fear into positive forms, and is said to facilitate meditation by allowing for a heightening of awareness, bringing greater mental clarity and relaxation. Whether or not you choose to believe that is of course up to you -- but merely allowing the suggestion to imprint itself on your mind can transform this simple rock (and yourself) into a powerful tool you can use to facilitate your journey towards mental, physical, and spiritual evolution. Or somethin'.