The Varieties of Silica:
Quartz, Chalcedony & Opal

Quartz, chalcedony and opal are all forms of silica (SiO2). The difference is that quartz has visible crystals whereas chalcedony crystals can only be seen under a microscope and opal has no crystals at all.

Quartz is, by far, the most common mineral on earth. 

There are hundreds of varieties of quartz, chalcedony and opal, mostly distinguished by their color.

Some of the more common varieties of quartz are:

• Amethyst
  Purple quartz
• Aventurine
  An opaque form of quartz containing small inclusions of shiny minerals which give a glistening effect. It is usually green but may also be yellow, orange or brown.
• Blue Quartz
  Dark blue, opaque quartz.
• Blue Rock Crystal
  Transparent crystalline quartz tinted pale blue by irradiation.
• Citrine
  Yellow orange or brown quartz
• Drusy Quartz
  Quartz with prismatic crystals protruding from a cavity or wall 
• Ghost Quartz
  Quartz in which one crystal forms over another giving the appearance of a "ghost" of the inner crystal
• Milky Quartz
  White translucent or opaque quartz
• Prase
  Quartz colored green by the presence of inclusions of green minerals
• Prasiolite
  Olive green quartz produced by heating amethyst or citrine
• Rock Crystal
  Colorless, transparent crystalline quartz.
• Rose Quartz
  Pink quartz
• Scepter Quartz
  Quartz crystals with a deformity in which the top part of one crystal protrudes from the rest
• Smoky Quartz
  Black or dark brown quartz
• Twin Quartz
  Quartz in which the crystals growth together in a regular manner, such as at right angles to each other (called a "Japanese twin")

Some of the common varieties of chalcedony are:

• Agate
  A banded form of chalcedony. It occurs in almost every imaginable color and pattern.
• Carnelian
  Reddish chalcedony.
• Chrysoprase
  Green chalcedony
• Jasper
  Opaque chalcedony of any color
• Onyx
  Agate with  evenly spaced, straight, parallel bands
• Plasma
  Dark green chalcedony
• Sard
  Brownish or brownish-red chalcedony
• Sardonyx
  Agate with straight brownish or brownish-red bands

Some opal displays different colors when viewed from different directions. Any opal which exhibits this play of colors is called "precious opal"; opal which does not exhibit a play of colors is called "common opal". Some of the common varieties of opal are:

• Banded Opal
  Common opal with colored bands. (Sometimes called "agate opal").
• Bandfire Opal
  Precious opal with a play of colors in bands
• Black Opal
  Precious opal with a dark-colored background. It may be black, dark gray, dark blue or dark green.
• Boulder Opal
  Precious opal found in sandstone or ironstone boulders.
• Crystal Opal
  Transparent or translucent precious opal
• Flame Opal
  Precious opal with red streaks which flicker when the stone is rotated
• Fossil Opal
  A fossil in which the organic material has been replaced by opal
• Gold Opal
  Gold colored common opal
• Honey Opal
  Transparent or translucent, common or precious opal with a honey-colored background
• Lemon Opal
  Lemon-yellow colored common opal
• Milky Opal
  Milky-white common or precious opal
• Mother of Opal
  Precious opal with bright colored specks
• Opalite
  Imitation or impure opal
• Pinpoint (or Pinfire) Opal
  Precious opal with tiny color flashes
• Pipe Opal
  Opal formed in long cylinders in rock
• Prase Opal
  Green-colored common opal
• Quinzite
  Rose- or pink-colored common opal
• Rainbow Opal
  Precious opal with curved bands of color
• Resin Opal
  Red-brown colored common opal
• Wax Opal
  Yellow- or brown-colored common opal with a waxy luster
• White Opal
  Precious opal with a light-colored background.