My Mental Rock Collection

Via Geotripper, I have learned that today is "Collect Rocks Day." OK. These are part of my mental collection: rocks I can't take home, but I know where they are and point them out to others from time to time. The following two photos are of the same fragment of opal, brightly colored (I presume) by iron oxides. No scale, but the pebble is about an inch (2.5 cm) in longest dimension.The glassy luster, conchoidal fracture and associated rocks (predominantly volcanic) are diagnostic. Microcrystalline quartz (agate, jasper, flint, chert, etc.) would have similar fracture, but more ragged; the luster of such material would be more waxy than glassy along a fresh fracture.
I'm pretty sure that whatever "play of light," or schiller, is visible in the above two photos results from internal fractures that create constructive and destructive interference in the reflected light.

The opal in the next two photos is about 0.4 inches (1 cm) long in its longest dimension, and does seem to have a little of the opal-like, gemmy schiller.
If you click to enlarge the pictures, you can also see the banding formed when the opal precipitated from solution. This texture and pattern would typically lead to the description of the pebble as agate, but like so many geological terms, the name agate has textural (how things are arranged), genetic (how it formed), and compositional (what it's made of) implications. "Agate" would work in terms of the texture and genesis of this pebble, but agate is banded chalcedony. Chalcedony is a fibrous form of microcrystalline quartz, SiO2. Opal is amorphous (no orderly arrangement of atoms at a molecular level) SiO2*H2O. I describe these banded opals as opaline agates to specify their composition.
Earlier this year I started looking fairly closely at the pebbles in the sidewalk outside my favorite coffee shop, and have been very pleasantly surprised at some of the wonderful clasts I've found, and particularly thrilled by the amount of regional and local geology one might learn by looking carefully at what's under your feet almost anywhere you go. No matter how far away the nearest outcrop is.