What kind of rock is this?
Updated
On this page8
Decide first which of the three families your rock belongs to, then look at its grains to name it. Igneous rocks cooled from melted rock and are made of interlocking crystals or glass. Sedimentary rocks formed at the surface from grains, shells or chemical deposits and usually show layers. Metamorphic rocks are older rocks changed by heat and pressure without melting, and often have a banded or sheet-like grain called foliation [1]. A hand lens, a steel nail, a piece of glass and the geologic map of the spot where you found it will name most of the rocks a beginner picks up. Below: what to look for, the common rocks, and reading a state geologic map.
Three questions that sort most rocks
| Look for | If yes | If no |
|---|---|---|
| Interlocking crystals, or glass, with no layering or rounded grains | Probably igneous | Go on |
| Rounded or broken grains cemented together, layers, or fossils | Probably sedimentary | Go on |
| Flat, shiny mineral flakes lined up in sheets or bands, or a hard, sugary rock made of one recrystallized mineral | Probably metamorphic | Look again on a fresh break, with a lens |
Look on a freshly broken surface, not the weathered outside, which hides grains under a rind of stain and soil. The U.S. Geological Survey's collector's leaflet recommends a hand lens of six to ten power for seeing the grains [1].
Igneous rocks: cooled from melt
The USGS leaflet defines igneous rocks as those "formed from molten material that has cooled and solidified" [1]. How fast it cooled controls the grain you see:
- Slow cooling, deep underground, gives large crystals, "a coarse-grained rock" in the leaflet's words [1]. You can see the individual minerals.
- Fast cooling at or near the surface gives small crystals, so "a fine-grained rock results" [1]. You need a lens or cannot see grains at all.
- Very fast cooling gives glass with no crystals.
- Gas bubbles caught in lava leave holes (vesicles), which later minerals may fill.
Common igneous rocks a beginner finds
| Rock | What you see | Grain |
|---|---|---|
| Granite | Speckled pink, white, gray and black; you can pick out glassy quartz, blocky feldspar and dark mica | Coarse |
| Basalt | Dark gray to black, heavy, often with gas holes | Fine |
| Andesite porphyry | Fine gray groundmass with larger, scattered crystals | Mixed: two sizes |
| Obsidian | Black or dark brown glass with curved, sharp fracture | None (glass) |
The USGS leaflet names obsidian, granite, basalt and andesite porphyry as igneous rocks [1]. Obsidian has a lookalike, glass slag from smelting, which you separate by where it was found; see common lookalikes. Small rounded obsidian nodules are sold and collected as "Apache tears," and BLM Arizona's rockhounding brochure lists them among specimens found on ground managed by its Kingman and Hassayampa field offices [2].
Sedimentary rocks: made at the surface
The USGS leaflet describes sedimentary rocks as "formed at the surface of the Earth, either in water or on land," usually as layered accumulations of sediment cemented together [1]. They are the rocks most likely to hold fossils and geodes.
| Rock | What you see | Key test |
|---|---|---|
| Sandstone | Sand-sized grains cemented together; gritty to the touch; often tan, red or gray | Rub it: grains come loose; under a lens, rounded grains |
| Shale | Fine, smooth, splits into thin flat layers | Too fine to see grains; feels smooth, not gritty |
| Conglomerate | Rounded pebbles in a finer cement | Visible rounded clasts |
| Limestone | Gray, tan or white, often with shells or shell fragments | Made mostly of calcite, so a steel nail scratches it [3] |
| Chert | Hard, dense, smooth, breaks with curved fracture | Scratches glass, as quartz does [3] |
Limestone is mostly calcite, which has a hardness of 3, while sandstone grains are mostly quartz at 7 [3][4]. That gives you a quick field check without acid: a nail grooves limestone easily and only rattles off the quartz grains of sandstone.
Jasper and agate as rock-forming silica
Jasper and agate are forms of microcrystalline quartz (silica) that fill cracks and cavities in other rocks or form beds and nodules of their own. Agate shows bands; jasper is opaque and usually red, brown or yellow. In sedimentary rocks, silica gel at the edge of a cavity can dehydrate into fine-grained chalcedony, with later layers growing inward to form agate [5]. They are hard (about 7) and scratch glass [3]. You will find them as pebbles in river gravels and on beaches, as nodules in limestone and volcanic ash, and as fillings in geodes and thundereggs.
Metamorphic rocks: changed without melting
Metamorphic rocks are existing rocks changed by heat and pressure deep in the crust. The USGS leaflet notes that metamorphism "does not melt the rocks" but produces denser, often foliated rock [1]. Foliation is the lining up of flat minerals such as mica into sheets or bands, which makes the rock split or look striped.
| Rock | What you see | Formed from |
|---|---|---|
| Slate | Dark, fine, splits into flat smooth sheets | Shale |
| Schist | Glittering, wavy sheets of mica you can see; often with garnets | Shale or other fine rock, further changed |
| Gneiss | Coarse light and dark bands | Granite or other rocks |
| Marble | Sugary, interlocking crystals; scratched by a nail | Limestone |
| Quartzite | Very hard, sugary; breaks through the grains, scratches glass | Sandstone |
Schist is a common host rock for garnet crystals. Marble and quartzite look alike and the nail test separates them, because marble is calcite (3) and quartzite is quartz (7) [3].
Using a state geologic map
A geologic map colors the ground by rock unit: each color or pattern is a body of rock of one kind and age, explained in the map's legend. The USGS leaflet calls geologic maps "unsurpassed as collecting guides," since they show where particular rocks occur and most include brief descriptions of them [1]. Before a trip, the map tells you what you will find; after a trip, it tells you what you probably picked up.
- Find the map. The U.S. Geological Survey's National Geologic Map Database catalogs published geologic maps, each with a product record and a "mapView" link to see other maps of the same area [6]. Your state geological survey publishes the state map and more detailed quadrangle maps, and many states offer them free online.
- Use the national compilation for a first look. The USGS State Geologic Map Compilation joins the state geologic maps into one database for the conterminous United States (Horton, San Juan and Stoeser, USGS Data Series 1052, 2017), and USGS products built on it group map units into 31 generalized rock types [7]. That generalizing is a reminder that a state-scale unit can contain several rock types; check the unit description.
- Locate your spot. Put the coordinates from your field notes on the map and read the unit there.
- Read the unit description. The legend or the accompanying text gives the rock types, sometimes the minerals, and the age. "Interbedded sandstone, shale and limestone" tells you the gray rock with shells is probably limestone.
- Check the scale. A statewide map generalizes heavily; one color may hide many small outcrops. A detailed quadrangle map is better for a single site.
A geologic map shows the rock and says nothing about who manages the land. Before you collect at a spot you found on the map, check the land status; finding out who manages the ground shows how.
When a rock is really a mineral, or a mix
Some things beginners call rocks are single minerals: a lump of quartz, a mass of calcite, a chunk of pyrite. If a piece looks like one material throughout, with one luster and one hardness, treat it as a mineral and test it as one. The words rock, mineral, crystal and gemstone, and where trade usage differs from the science, are defined on rocks, minerals, crystals and gemstones.
Naming the minerals inside the rock
Once you know the rock, the next question is usually the crystal or the colored grain in it. The order of tests that names a single mineral, starting with the locality, is on identifying a mineral.
References
- U.S. Geological Survey, Collecting Rocks, INF-64-4 (1976 printing). https://www.michigan.gov/documents/deq/GIMDL-USGSCR_302981_7.pdf
- Bureau of Land Management, Arizona, Rock Hounding in Arizona, BLM/AZ/GI-14/009 (undated brochure, read October 2026). https://BLM.GOV/sites/default/files/documents/files/2014%20Rockhounding_14_web_2018.pdf
- Michael Rygel and Page Quinton, Sedimentary Geology: Rocks, Environments and Stratigraphy (SUNY Potsdam), "14.1: Mineral Identification Tables," Geosciences LibreTexts, January 8, 2026, CC BY-SA 4.0. https://geo.libretexts.org/Courses/SUNY_Potsdam/Sedimentary_Geology%3A_Rocks_Environments_and_Stratigraphy/14%3A_Appendices/14.01%3A_Mineral_Identification_Tables
- Encyclopaedia Britannica, "Mohs hardness," revised and updated by Erik Gregersen, published 10 May 2025. https://britannica.com/print/article/387714
- EarthDate, "Potato Stones and Thunder Eggs," Episode 414, April 1, 2025. https://earthdate.org/node/1181
- U.S. Geological Survey, National Geologic Map Database, product record for Wolfe and McKee, Sedimentary and igneous rocks of the Grays River quadrangle, Washington, USGS Bulletin 1335 (1972), read October 2026 as an example NGMDB record. https://ngmdb.usgs.gov/Prodesc/proddesc_21444.htm
- U.S. Geological Survey, "Geology shapefiles for the United States and Australia," data.gov catalog record, last modified March 31, 2025, doi:10.5066/P970GDD5 (citing Horton, San Juan and Stoeser, 2017, USGS Data Series 1052). https://catalog.data.gov/harvest_record/f7c8a1d6-2c9c-446a-8d5f-adcdd2124af9/raw
