Rock Identification
"On this page is some basic help with rock identification.."
First Step
First, decide whether the rock you have picked up is a sedimentary, metamorphic, or igneous rock.
An igneous rock has got an arrangement of mineral crystals. It has got no foliation, and no fossils.
Granite - a typical igneous rock with interlocked mineral
grains.
A sedimentary rock can be identified by visible sedimentary layers. But not all sedimentary rocks do have sedimentary layers. They do, however, have a special look - fine-grained ones have a very homogenous structure, others contain larger pebbles or fossils. They are not foliated. Sedimentary rocks can be quite weak, you can try its hardness by hitting it with a hammer.
A typical, "sandy" sedimentary rock with fossil.
A metamorphic rock is often foliated. If it's not, it is still different from igneous rocks because it does not have mineral crystals; and it differs from sedimentaries because it lacks bedding layers, fossils, and the sandy, homogenous structure.
A typical "shisty" (layered/foliated) metamorphic rock.
Rock Identification - Second Step
Once you know what type of rock it is, establish whether it is coarse, medium or fine grained. A rock is coarse-grained if you can see its grains with your bare eyes. With medium-grained rocks, you need a hand lens to see the grains separately. And with a fine-grained rock you need a microscope.
Rock Identification - Third Step
If you rock was an igneous rock, look at its colour.
| Coarse Grained | Medium Grained | Fine Grained | |
| Light | Granite | Quartz porphyry | Rhyolite |
| Medium | Diorite, Syenite | Andesite Porphyry, Monzonite | Andesite, Trachyte |
| Dark | Gabbro | Dolerite | Basalt |
Basalt - typical mafic igneous rock. By Dave Schumaker
If you rock was a sedimentary rock, look at what minerals you find in the rock.
| Coarsest Grained | Medium Grained | Finest Grained | |
| Rock Fragments | Conglomerate | Breccia | Greywacke |
| Quartz | Sandstone | Mudstone | Siltstone, Shale |
| Carbonates | Limestone | Travertine | Chalk, Marl |
| Other Minerals | Ironstone, Dolomite | Rock salt, Rock Gypsum | Flint, Chert, Coal, Clay, Amber |
Conglomerate - coarse-grained sedimentary rock. By Molas
If you rock was a metamorphic rock, look at whether it is foliated or not.
| Coarse Grained | Medium Grained | Fine Grained | |
| Foliated | Eclogite, Amphibolite, Migmatite, Gneiss | Schist, Phyllite | Slate |
| Not Foliated | Skarn, Marble, Granulate | Hornfels, Marble, Slate | Mylonite, Halleflinta |
Gneiss - a typical metamorphic rock. By xrichx
