Another name for this semi-precious gemstone is Merlinite. While opalite isn't really opal, it is a molecular cousin of the precious variety. Opalite typically doesn't have any of the dazzling color/fire associated with well-known precious opal, and is much more common.
What makes this opalite unique is the fern-like dendrites that spider through the rock. You never know what patterns or scenes will emerge until you cut into it. It is very hard and takes a bright polish.