Rock Identification Tables
These tables will help you identify almost any rock you're likely to find. First, decide whether your rock is igneous, sedimentary or metamorphic.
- Igneous: A tough, frozen melt with little texture or layering; mostly black, white and/or gray minerals; may look like lava (about igneous rocks)
- Sedimentary: Hardened sediment with layers (strata) of sandy or clayey stone; mostly brown to gray; may have fossils and water or wind marks (about sedimentary rocks)
- Metamorphic: Tough rock with layers (foliation) of light and dark minerals, often curved; various colors; often glittery from mica (about metamorphic rocks)
Next, check the rock's grain size and hardness. Then start in the left column of the appropriate table below and work your way across. Follow the links to pictures and more information. If you don't find a match, try another of the three big types.
Grain Size: "Coarse" grains are visible to the naked eye (greater than about 0.1 millimeter), and the minerals can usually be identified using a magnifier; "fine" grains are smaller and usually cannot be identified with a magnifier. (using a magnifier, identifying minerals)
Hardness: Hardness (as measured with the Mohs scale) actually refers to minerals rather than rocks, so a rock may be crumbly yet consist of hard minerals. But in simple terms, "hard" rock scratches glass and steel, usually signifying the minerals quartz or feldspar (Mohs hardness 6-7 and up); "soft" rock does not scratch a steel knife but scratches fingernails (Mohs 3-5.5); "very soft" rock does not scratch fingernails (Mohs 1-2). Igneous rocks are usually hard.
Identification of Igneous Rocks
| Grain Size | Usual Color | Other | Composition | Rock Type |
| fine | dark | glassy appearance | lava glass | Obsidian |
| fine | light | many small bubbles | lava froth from sticky lava | Pumice |
| fine | dark | many large bubbles | lava froth from fluid lava | Scoria |
| fine or mixed | light | contains quartz | high-silica lava | Felsite |
| fine or mixed | medium | between felsite and basalt | medium-silica lava | Andesite |
| fine or mixed | dark | has no quartz | low-silica lava | Basalt |
| mixed | any color | large grains in fine-grained matrix | large grains of feldspar, quartz, pyroxene or olivine | Porphyry |
| coarse | light | wide range of color and grain size | feldspar and quartz with minor mica, amphibole or pyroxene | Granite |
| coarse | light | like granite but without quartz | feldspar with minor mica, amphibole or pyroxene | Syenite |
| coarse | medium to dark | little or no quartz | low-calcium plagioclase and dark minerals | Diorite |
| coarse | medium to dark | no quartz; may have olivine | high-calcium plagioclase and dark minerals | Gabbro |
| coarse | dark | dense; always has olivine | olivine with amphibole and/or pyroxene | Peridotite |
| coarse | dark | dense | mostly pyroxene with olivine and amphibole | Pyroxenite |
| coarse | green | dense | at least 90% olivine | Dunite |
| very coarse | any color | usually in small intrusive bodies | typically granitic | Pegmatite |
Identification of Sedimentary Rocks
| Hardness | Grain Size | Composition | Other | Rock Type |
| hard | coarse | clean quartz | white to brown | Sandstone |
| hard | coarse | quartz and feldspar | usually very coarse | Arkose |
| hard or soft | mixed | mixed sediment with rock grains and clay | gray or dark and "dirty" | Wacke/ Graywacke |
| hard or soft | mixed | mixed rocks and sediment | round rocks in finer sediment matrix | Conglomerate |
| hard or soft | mixed | mixed rocks and sediment | sharp pieces in finer sediment matrix | Breccia |
| hard | fine | very fine sand; no clay | feels gritty on teeth | Siltstone |
| hard | fine | chalcedony | no fizzing with acid | Chert |
| soft | fine | clay minerals | foliated | Shale |
| soft | fine | carbon | black; burns with tarry smoke | Coal |
| soft | fine | calcite | fizzes with acid | Limestone |
| soft | coarse or fine | dolomite | no fizzing with acid unless powdered | Dolomite rock |
| soft | coarse | fossil shells | mostly pieces | Coquina |
| very soft | coarse | halite | salt taste | Rock Salt |
| very soft | coarse | gypsum | white, tan or pink | Rock gypsum |
Identification of Metamorphic Rocks
| Foliation | Grain Size | Hardness | Usual Color | Other | Rock Type |
| foliated | fine | soft | dark | "tink" when struck | Slate |
| foliated | fine | soft | dark | shiny; crinkly foliation | Phyllite |
| foliated | coarse | hard | mixed dark and light | wrinkled foliation; often has large crystals | Schist |
| foliated | coarse | hard | mixed | banded | Gneiss |
| foliated | coarse | hard | mixed | distorted "melted" layers | Migmatite |
| foliated | coarse | hard | dark | mostly hornblende | Amphibolite |
| nonfoliated | fine | soft | greenish | shiny, mottled surface | Serpentinite |
| nonfoliated | fine or coarse | hard | dark | dull and opaque colors, found near intrusions | Hornfels |
| nonfoliated | coarse | hard | red and green | dense; garnet and pyroxene | Eclogite |
| nonfoliated | coarse | soft | light | calcite or dolomite by the acid test | Marble |
| nonfoliated | coarse | hard | light | quartz (no fizzing with acid) | Quartzite |
