Rock & Mineral Identification Key

A mineral is a naturally occurring, inorganic solid with a specific chemical composition and crystalline structure (e.g., quartz, feldspar, calcite). A rock is an aggregate of one or more minerals (e.g., granite contains quartz, feldspar, and mica). Think of minerals as the "ingredients" and rocks as the "cake."

Use the Mohs Hardness Scale with common household items: your fingernail (~2.5), a copper penny (~3.5), a steel nail or knife blade (~5.5), and a piece of glass (~5.5). If the mineral scratches the item, it's harder. If the item scratches the mineral, the mineral is softer. Window glass is a great benchmark at hardness 5.5.

A streak test involves rubbing the mineral across an unglazed porcelain tile (a streak plate). The color of the powdered streak is often more reliable than the mineral's surface color for identification. For example, hematite may look silvery-gray but always produces a distinctive reddish-brown streak. Pyrite (fool's gold) leaves a greenish-black streak, while real gold leaves a golden-yellow streak.

Metallic luster looks like polished metal (shiny, reflective, opaque). Examples include pyrite, galena, and hematite. Non-metallic luster includes vitreous (glassy like quartz), pearly (like mica), greasy, silky, or dull/earthy appearances. A good rule of thumb: if it looks like a piece of metal, it has metallic luster. If it looks more like glass, plastic, or ceramic, it's non-metallic.

Cleavage is the tendency of a mineral to break along flat, parallel planes determined by its crystal structure. Minerals like mica have perfect cleavage in one direction (peeling into sheets). Calcite has cleavage in three directions forming rhombohedrons. Feldspar has two cleavage planes at nearly 90°. Quartz, by contrast, has no cleavage and breaks with a conchoidal (shell-like) fracture.

Yes! White vinegar (acetic acid) can substitute for dilute hydrochloric acid in a simple acid test. Place a drop of vinegar on the mineral. If it fizzes or bubbles, the mineral likely contains calcium carbonate — most commonly calcite or limestone. Dolomite will only fizz when powdered. This is a safe and accessible test for home use.

Magnetite is the most strongly magnetic common mineral — a regular refrigerator magnet will attract it. Pyrrhotite is also magnetic but less common. Some hematite specimens show weak magnetism. If your mineral strongly attracts a magnet, it's very likely magnetite. This is one of the quickest and most definitive field tests available.

Igneous rocks often have interlocking crystals (granite) or may contain gas bubbles (basalt, pumice). Sedimentary rocks typically show layers, may contain fossils, and often have visible grains cemented together (sandstone, limestone, shale). Metamorphic rocks show banding or foliation (gneiss, schist, slate) and crystals may be stretched or aligned. The presence of fossils almost always indicates a sedimentary rock.

Quartz is by far the most commonly found mineral — it's abundant, hard (scratches glass), and comes in many varieties (amethyst, citrine, rose quartz, smoky quartz). Other common finds include feldspar (often pink or white with flat cleavage surfaces), calcite (fizzes with acid, soft), mica (peels into shiny flexible sheets), and pyrite (metallic gold cubes, "fool's gold"). In rocks, granite and basalt are extremely common worldwide.

Mineral appearance varies greatly due to impurities, weathering, crystal habit, and locality. For example, quartz can be clear, white, pink, purple, brown, or even black. Surface weathering can dull a mineral's luster. Always rely on physical properties (hardness, streak, cleavage, specific gravity) rather than color alone for reliable identification. Experienced geologists prioritize these objective tests over visual appearance.