Reading the landscape: where certain materials tend to turn up

Certain geologic settings consistently produce certain kinds of material, and learning to recognize them saves a lot of wasted searching. Alluvial gravel washes and old riverbeds concentrate durable, dense material through natural water sorting, which is why they are classic spots for tumbled agate and jasper as well as gold and other heavy minerals. Volcanic fields with rhyolite flows are the classic host rock for thundereggs and geodes, which form when gas cavities in the cooling lava are later filled in by silica rich groundwater. Pegmatite dikes, coarse grained granite intrusions that cooled slowly enough to grow large crystals, are the textbook source for gem quality tourmaline, beryl, and large quartz crystals. Contact zones where an intrusion met surrounding rock can produce garnet and other skarn minerals through localized metamorphism. Historic mine dumps and tailings sometimes hold overlooked material that was not valuable enough to process at the time, but these are almost always on claimed or private property, so permission from the landowner or claim holder is not optional.