Geo 730: March 13, Day 437: Opal and Chalcedony

Dana commented repeatedly, about this stop, that on approach and drive-by, it just looks like a pile of uninteresting, dull gray rock. And it's true: not until you get your nose up against the outcrop do you see the brilliant splashes of color. Most of the wild coloring is in opal; the chalcedony/agate tends to be either white or colorless and translucent. Opal is amorphous (non-crystalline) hydrated silica, while chalcedony is microcrystalline quartz, most often with a fibrous habit that may or may not be visible to the naked eye, but can be made out clearly in a microscope. Under the scope, you can't resolve the individual crystals, but you can see their orientations en mass. Opal tends to have a vitreous luster, while chalcedony has a more waxy luster on broken surfaces. And "agate" is a term that is more useful to collectors of pretty rocks, but to geologists simply means color-banded chalcedony.

Under the chisel tip of the hammer, another lithophysa is visible, with an inner rim of vapor-phase silica, and above the head is a patch of white agate, with a brilliant patch of reddish-orange opal to the left. None of the opal here is gemmy- that is, it doesn't have the shimmering play of color that one would see in jewelery opal. There is gem-quality opal in the area, but sites where it occurs are mostly claimed (there are quite a few pay-to-dig sites in Virgin Valley), and the gemmy material is mostly opaline replacement of wood. Finally, spherulites are abundant throughout the mass.

Note: If you decide to visit this spot, please see Tuesday's comments on safety issues.

Photo run through Paint.Net's autolevel routine for contrast and saturation. August 19, 2011. FlashEarth Location.