Re: Help Identify These Rocks and Minerals of Southern Nevada

Ben, some of your specimens COULD be partly opal too. This is something that can take a lot of experience to distinguish–seeing real specimens (or looking at a lot of good, representative pictures–but real rocks in person is best) is the best way to learn to recognize such things; but even then, “experts” can be uncertain. If crystals or crystal faces are visible, or if its overall just “sparkly, it is crystalline quartz. If the material is solid, no crystals evident, transparent to translucent, then it’s agate (if banded) or just chalcedony (none or very little banding evident). Opal is more glassy-looking, also without any crystal form. “Hyalite” opal is completely clear and glassy, usually as thin layers coating other minerals or rock surfaces. [Hyalite is from “hyaline”, an adjective which means, glassy.] Common opal, sometimes also called “opalite”, without any fire, is massive, sometimes chalk-like or porcelain-like, but still has more of a shiny, glass-like (but, opaqu…
FMF – Friends of Minerals Forum, discussion and message board