How do I tell apart minerals that look the same?

Most confusions between lookalike minerals are settled by one well-chosen test, usually hardness, streak or heft, run on a hidden spot. Quartz scratches glass and calcite does not. Pyrite is hard and brittle with a greenish-black streak, while gold is soft, bends and streaks gold. Fluorite has four directions of cleavage and quartz none [1][2][3]. A few pairs, such as natural obsidian and glass slag, cannot be told apart reliably by hand at all, and the honest answer is the locality or a lab. This page takes the common pairs one at a time: what you see, the test that separates them, and what to do when that test is not enough.

The quick table

Quartz or calcite

Both are often colorless or white with a glassy luster, both form pointed crystals, and both are among the most common minerals a collector finds. The separation is easy:

• Hardness. Quartz is 7 on the Mohs scale and calcite 3 [1]. Draw an edge of the crystal across a glass plate. Quartz bites in and leaves a groove; calcite skates over it or leaves powder that wipes away. A steel nail scratches calcite and slides off quartz.
• Cleavage. Calcite breaks into rhombs, with cleavage in three directions not at 90 degrees. Quartz has no cleavage and breaks with a curved, glassy fracture [3].
• Crystal shape. Quartz usually forms six-sided crystals [3]; calcite takes many forms, including rhombs, blades and pointed "dogtooth" crystals.
• Double image. Clear calcite shows double refraction: a line seen through a cleavage rhomb appears doubled [3].

About the acid test

University tables note that calcite "fizzes strongly in dilute HCl" while dolomite fizzes only when powdered [3]. That is a real identification property and geology labs use it. On a specimen you want to keep, acid etches the surface it touches, so acid and vinegar have no place on a carbonate specimen you want to keep. You do not need them here: hardness and cleavage separate quartz from calcite, and when a fine-grained carbonate has to be pinned down (calcite or dolomite, for example), a lab can do it without touching the show surface.

Gold, pyrite or chalcopyrite

All three are brassy-yellow and metallic, and "fool's gold" has fooled people for as long as there have been prospectors. Pyrite and chalcopyrite are very common; gold in visible pieces is not.

The fastest test is the streak plate. If the powder is black or greenish, it is not gold. If you would rather not powder it, press a steel pin into a small grain: gold dents and smears like soft metal, pyrite does not yield, and chalcopyrite scratches and powders. The MSA key adds that chalcopyrite is "distinguished from pyrite by softness and crystal shape" [2].

Pyrite and chalcopyrite are sulfides. National Park Service guidance notes that pyrite in humid conditions can release sulfur dioxide, and that chalcopyrite and other sulfides can give off gases under some conditions [5]. Keep them dry once identified.

Fluorite or quartz

Fluorite comes in purple, green, blue, yellow and clear, as does quartz, and both are glassy. Fluorite is 4 and does not scratch glass; the SUNY Potsdam tables put it plainly: it "resembles quartz but does not scratch glass" [3]. Fluorite also has good cleavage in four directions, which breaks it into octahedral pieces, and often grows as cubes. Quartz has no cleavage and forms six-sided prisms [3]. A light knife scratch on the back of a fluorite cube marks it; on quartz it does nothing.

Obsidian or glass slag

Obsidian is natural volcanic glass, an igneous rock that cooled too quickly to grow crystals [6]. Slag is man-made glass left by smelting and glass making. Both are glassy, both break with curved, shell-like fracture, and both sit near the hardness of window glass (5.5) [1]. No hand test on this page reliably separates them, and you should be suspicious of any site that says one does.

What does help:

• Locality. Obsidian occurs in volcanic country, in or near rhyolite flows. Slag turns up near old smelters, furnaces, railroads, glassworks and dumps, and on beaches and riverbanks far from any volcano. A geologic map of the spot answers the question more often than the specimen does [6].
• Context of the find. A field of black glass pebbles weathering out of volcanic rock is obsidian. A single bright blue or green glassy lump in a creek below a town is far more likely slag.
• A lab. X-ray diffraction identifies crystalline minerals by their patterns [7], so it shows that a glass is glass but not whose glass it is. A chemical analysis can separate natural from industrial compositions; ask the lab whether it can.

Glass and slag are also sold under trade names and as "obsidian" in the decorative trade; the treated and imitation stones of that trade are covered on dyed, heated, coated and lab-grown stones.

Garnet or another red stone

Red garnets are common in metamorphic rocks and river gravels. The candidates that get confused with them include red glass, cuprite and cinnabar in collections, and corundum (ruby) in gem gravels.

• Hardness. Garnet is 6½ to 7½ and scratches glass [3]. Cuprite (3½ to 4) and cinnabar (2½) do not [8]. Corundum is 9 and scratches garnet [1].
• Heft. Garnet's specific gravity is 3.1 to 4.3 [3]; the Maine Geological Survey gives 3.5 to 4.3 [9]. A garnet feels heavier than a quartz pebble of the same size.
• Streak. Garnet gives a white streak (or none, being harder than many plates) [3]; cuprite and cinnabar give red streaks [8].
• Habit. Garnets often form ball-like crystals with many faces.

Cinnabar is a mercury mineral; do not powder or handle it bare-handed [5].

Agate or chalcedony

This pair is a matter of names more than minerals. Chalcedony is the general name collectors use for fine-grained, microcrystalline quartz that forms crusts, linings and nodules. Agate is chalcedony with visible banding. Jasper is the opaque, usually red, brown or yellow, form colored by impurities. The EarthDate program describes how silica gel at the edge of a cavity dehydrates into fine-grained chalcedony and how later layers "can grow inward, sometimes forming agate" [10]. All of them have quartz's hardness of about 7 and scratch glass [1][3].

So the question to ask a piece is whether it shows bands. If it does, collectors call it agate; if it is translucent and even, chalcedony; if opaque and colored, jasper. Dyed agate is common in the decorative trade, and colors that look too bright usually are; see dyed, heated, coated and lab-grown stones.

Rocks that look like meteorites

Heavy, dark, rusty or magnetic rocks are often mistaken for meteorites. Magnetite (black, magnetic, black streak) and hematite (red-brown streak) are the usual suspects [3]. The checks for that question, and how to get a suspected meteorite classified, live on is it a meteorite?

When the pair will not separate

Some lookalikes need instruments: calcite and dolomite in fine-grained masses, the members of mineral groups such as garnets or feldspars, and glass of uncertain origin. A Raman study of semiprecious stones published in Spectroscopy found that even Raman could not separate synthetic corundum from natural corundum, and that emerald and aquamarine, both beryl, give similar spectra [11]. What each method can and cannot tell you, and what named labs charge, is on getting a specimen tested.

References

1. Encyclopaedia Britannica, "Mohs hardness," revised and updated by Erik Gregersen, published 10 May 2025. https://britannica.com/print/article/387714
2. Mineralogical Society of America, The Mineral Identification Key, Table IB: Minerals with metallic or submetallic luster and hardness greater than 2½ but less than 5½ (undated, read October 2026). https://msaweb.org/msa/Collectors_Corner/id/mineral_id_keytib.htm
3. 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
4. Mesa Community College, Robert S. Leighty, GLG 101IN Lab 4: Minerals, Part D: Economic Minerals (undated, read October 2026). https://physci.mesacc.edu/Geology/Luther/GLG101IN/GLG101IN_Lab04_Minerals/GLG101IN_Lab04_Minerals5.html
5. National Park Service, Greg McDonald, "Health and Safety Issues with Geological Specimens," Conserve O Gram 11/11, September 2006. https://www.nps.gov/subjects/museums/upload/11-11_508.pdf
6. U.S. Geological Survey, Collecting Rocks, INF-64-4 (1976 printing). https://www.michigan.gov/documents/deq/GIMDL-USGSCR_302981_7.pdf
7. Northern Illinois University, Department of Earth, Atmosphere and Environment, Micro-Compositional Analysis Laboratory, "X-ray Diffraction (XRD) Siemens D-500 With Jade Software" (undated, read October 2026). https://www.niu.edu/earth-atmosphere-environment/research/labs/micro-compositional/x-ray-diffraction.shtml
8. Mineralogical Society of America, The Mineral Identification Key, Table IIA: Minerals with a nonmetallic luster, definitely colored streak, and hardness 1 to 6 (undated, read October 2026). https://msaweb.org/msa/Collectors_Corner/id/mineral_id_keytiia.htm
9. Maine Geological Survey, "Activity 19: The Density of Minerals," developed by Duane Leavitt (undated, read October 2026). https://www.maine.gov/dacf/mgs/education/lessons/act19.pdf
10. EarthDate, "Potato Stones and Thunder Eggs," Episode 414, April 1, 2025. https://earthdate.org/node/1181
11. S. Lowry, D. Wieboldt, D. Dalrymple, R. Jasinevicius and R.T. Downs, "The Use of a Raman Spectral Database of Minerals for Rapid Verification of Semiprecious Gemstones," Spectroscopy 24(5), May 1, 2009. https://www.spectroscopyonline.com/view/use-raman-spectral-database-minerals-rapid-verification-semiprecious-gemstones