Black opal: Flashes and speckles appear against black background
Water opal: A transparent, colorless opal is the background for brilliant flashes of color
Fire opal: Reddish or orange opal
Peridot [Olivine] (hardness: 7 Mohs)
Magnesium iron silicate
Specific gravity: 3.22-3.45
Olive to lime green
Quartz (hardness: 7 Mohs)
Silicon dioxide or silica
Specific gravity: 2.65