What is Quartz?

Quartz has no cleavage and fails by brittle, conchoidal fracture; the fracture surfaces have vitreous luster.

Quartz in thin section
 

Varieties

sometimes carved (quartz crystal)
used in optical devices, etc. (like radio frequency oscilltors)
can be doubly terminated

comes in different shades of purple (due to Fe impurity, commonly with zoned distribution):
light purple
purple
blue-purple
reddish purple
it is also set as a cabachon
radiation damage and iron (Fe) impurity are necessary to produce amethyst color
Fe - O charge transfer gives color: O-- <=> Fe4+
Fe3+ in Si site -> Fe4+
Other comments

transparent, shades of orange
Citrine
yellow-orange-brown
color may be zoned
the color is due to the presence of Fe, which is also the impurity present in amethyst.

The term "ametrine" refers to bi-colored citrine/amethyst (junction is a peach color).

smoky quartz is Al+++- bearing quartz that has been exposed to radiation (natural and unnatural)
it removes an electron from O-- (i.e., a color center effect)
if there is sufficient Al and radiation, the crystal can turn completely black! ("morian")
Heat treatement restores clarity
Also sometimes faceted

has rose-pink color
the color is often due to the element Titanium (Ti)
used for beads etc.
sometimes you can find star rose quartz which has inclusions
often massive (aggregates of crystals, not single crystals)

Milky quartz

milky appearance due to inclusions (often of fluid)
often associated with gold deposits - used as a gem mainly if gold present

Rutilated quartz
common in jewelry
The term "venus hair" refers to fine red rutile needles
Note the orientations of the rutile needles

examples of quartz containing tourmaline and Fe-oxide
fine tourmaline needles
a second example of green tourmaline in quartz
other inclusions can involve minerals (possibly rutile) and open tubes, for example:
at low magnification these features make interesting patterns,
the three dimensional nature of these needles is quite apparent
 

due to the presence of needles of asbestos etc. Form when prexisting minerals are replaced by quartz ('fossilized')
(tiger's eye): brown/yellow, and red
if needles retain their green asbestos color: "hawk's eye"; "falcon's eye"... etc.

Adventurine: green quartz containing platy inclusions of mica; can be used as a substitute for jade; used in beads etc.
 

Where does it come from ?

• Quartz is a very common mineral in most igneous, metamorphic, and sedimentary rocks. It has also been found in some lunar rocks and meteorites.
• Quartz forms in rocks of igneous origin (melts) only after other silicates have incorporated most other available cations.
• Rocks that are more than about 47 percent (by weight) silicon dioxide contain quartz. The quartz content increases as the weight percentage SiO2 increases.
• Quartz can be dissolved in hot water or steam and is thus transported from place to place in the Earth, being deposited by cooling of the transporting fluid or by release of pressure.
• Because quartz is relatively resistant to mechanical abrasion, it is abundant in stream sediments, on beaches, and in wind-blown sands. Quartzite and sandstone are mostly quartz because this is the most abundant mineral that survives processes occurring at the Earth's surface.

Synthesis

• Quartz was first synthesized in 1845 via a hydothermal method  > 1 lb. crystals
• Amethyst can be synthesized; distinguished mostly by "twinning"

Doped quartz (quartz containing an impurity such as cobalt (Co) may be synthesized to achieve bright colored materials.

Heat Treatment: Citrine and amethyst:

• before
• during
• after

Quartz has some interesting properties

What is it ?

• Tanzanite is the important gem variety of zoisite (zoisite is also refered to as epidote)
• Color is variable. Gem varieties are often sapphire blue or violet blue. However, other colors occur, like the red variety (thulite), and a green variety.
• Hardness: 6.5-7 (note this hardness is less than normally desirable for a gem used in jewelry subject to abrasion by dirt and dust)
• Crystal system: orthorhombic
• Composition: Ca-Al-silicate (tanzanite color is attributed to a Vanadium (V) as an impurity
• strongly pleochroic
• View an uncut crystal.

Where is it found?

• The variety of zoisite known as tanzanite was discovered in Tanzania in 1967
• it occurs in pegmatite veins (inclusions suggest hydrocarbon-rich fluids)

Other:

• usually heat treated at 400 - 500 C to remove brown tints, deepen blue
• some cats eye varieties due to channels or needles
• best orientation for color not same for chatoyancy