The rock collection.
Explore 34 rocks by how they form, what they look like, and the clues that set them apart.
Granite
Coarse-grained — individual mineral crystals are visible to the naked eye
Basalt
Fine-grained — crystals are too small to see individually, often looks almost glassy or dull
Diorite
Coarse-grained, crystals clearly visible
Gabbro
Coarse-grained, like granite or diorite, but dark
Peridotite
Coarse-grained
Rhyolite
Fine-grained, sometimes with a few larger crystals scattered in a fine matrix
Andesite
Fine-grained, medium gray to greenish-gray
Obsidian
Glassy luster, smooth
Pumice
Extremely light, full of visible air pockets/holes (vesicles)
Sandstone
Visibly grainy texture, feels gritty
Shale
Very fine-grained, smooth to the touch
Conglomerate
Visibly made of rounded pebbles/clasts embedded in a finer matrix
Siltstone
Fine-grained, but with a slight gritty feel — finer than sandstone, coarser than shale
Limestone
Usually light-colored: white, gray, tan, sometimes yellowish
Chalk
Very soft, fine, powdery — easily scratched with a fingernail
Rock Salt (halite)
Usually clear, white, or tinted pink/orange from impurities
Chert
Very hard (7 on the Mohs scale, like quartz) — will scratch glass
Coal
Black to dark brown
Slate
Splits into thin, flat, smooth sheets — even more cleanly than the shale it came from
Phyllite
Distinctive silky/satiny sheen (from the small aligned mica crystals) — this is the fastest way to identify it
Schist
Visible, large, sparkly mica flakes (or other platy minerals) aligned in wavy layers
Gneiss
Bold, coarse banding, alternating light (quartz/feldspar) and dark (mica/hornblende) layers
Marble
Crystalline, sugary-looking texture when broken
Quartzite
Very hard (7, like quartz) — resists scratching
Breccia
Made of visibly angular (sharp-edged, not rounded) rock fragments set in a finer matrix
Pegmatite
Extremely coarse-grained, individual crystals often large enough to pick out and identify by eye alone
Syenite
Coarse-grained, crystals clearly visible, same general look as granite at a glance
Tuff
Fragmental, often visibly made of compacted ash and small rock/pumice fragments
Scoria
Full of visible holes (vesicles), similar to pumice
Dolostone
Similar appearance to limestone: light-colored, fine to medium-grained
Gypsum
Very soft — can be scratched with a fingernail (2 on the Mohs hardness scale, one of the softest common rock-forming minerals)
Hornfels
Very fine-grained — individual crystals are far too small to see without a microscope
Amphibolite
Dark: black to dark green
Serpentinite
Green to greenish-black, sometimes mottled or streaked