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opal-common - no 'play' today
Another name for common opal is potch .
It's the orderliness of the silica spheres within the stone that separates precious opal from common opal. Precious opal is made up of tiny uniform spheres of transparent hard silica which fit together in an orderly three dimensional frame and sit in a "bath" of silica solution. But, in common opal, the silica spheres are in too much disarray to diffract light -- hence, no play-of-color.
When there is color -- that is, bits with "play of color" mixed with potch, it's called -- duh -- "potch and color."

silica spheres of precious opal vs. potch

Common opal is defined as lacking a "play of color" and includes a rather long list of stones, all of which lack opalescence, color or luster for the most part.

"potch and color"

Opaque or glassy in a wide range or colors, common opal can be sometimes have a waxy luster, and is often fluorescent. It may be colored or contain dendritic (moss-like) inclusions. There are many different common opals, including:
  • Hyalite is a transparent, colorless-to-bluish opal which forms in cavities in volcanic rocks.
  • Hydrophane is a porous, opaque, light-colored opal which becomes transparent when soaked in water.
  • Lemon (yellow) opal often has green-gold tones, small black and red-orange streaks, and a waxy luster.
  • Moss opal contains dendritic mineral inclusions, resembling ferns or moss.
  • Peruvian opal comes in shades of turquoise, aqua, and pale blue.
  • Pink opal is sometimes sold as pink silica and contains black inclusions and dendrites.
  • Prase opal is bright green, due to the presence of nickel.
  • Wood opal is a replacement of wood by non-precious opal, often retaining the grained appearance and bark texture.