agate on the left, a chert  concretion with an iron oxide core on the right Wouldn't it be nice to walk around knowing what kinds of rocks you're seeing, why they're there, how old they are, what story they're telling... and maybe even collecting some? I happen to live in a part of the world where the only rocks are pebbles found at the bottom of streams, and even still I managed to find the two rocks at the right when I went out looking for something nice to show you.


rock: a mass of solid mineral matter occurring naturally

Rocks are made of one or more minerals, and may include fossils.

Those rocks are shown about life-size. On the left in the picture you see an example of agate, and on the right is a chert concretion that's been broken open to reveal some jasper. Chemically, both of these rocks are mostly made of the mineral quartz, SiO2. However, the two rocks have different kinds of impurities in them, and have different histories, so they look amazingly different.

(using the Wentworth Size Scale)

clay.......less than 1/256 mm
silt.........1/256 to 1/16 mm
sand.............1/16 to 2 mm
granule.............2 to 4 mm
pebble.............4 to 64 mm
cobble...........64 to 256 mm
boulder......more than 256 mm

Well, there's a lot to know about rocks, and one way to start organizing your thoughts about them is to remember from your Sixth Grade lessons that there are three kinds of rock. To learn more about each of those three rock types, just click on their names:

  1. Igneous Rocks: examples: granite, basalt, diabase, diorite, gabbro,  obsidian, pumice, rhyolite, scoria
  2. Sedimentary Rocks: examples: limestone, sandstone, shale, breccia, conglomerate
  3. Metamorphic Rocks: examples: marble, gneiss, quartzite, schist, hornfels, slate

If you have a rock you need identifying, try Don Peck's Online Rock Key