Opal | #Geology
Formula: SiO2 • nH2O
Colour: Colorless, white, various
Hardness: 5½ - 6½
A pseudomorph (which means false shape in Latin) is a mineral that has replaced another mineral's chemistry or structure with its own without changing the outward shape of the original mineral while color, hardness, and other properties change to those of the replacing mineral. Transformations from one mineral to another are not unusual in nature, but preserving the outward shape of the original mineral is! The end result is that the crystal appears to be one mineral but is actually another.
Opal "Pineapples" - an opal pseudomorph formed by the replacement of glauberite by opal - found only in New South Wales.
Photo: Grant Pearson