What do streak and luster tell me about a mineral?
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Luster tells you how a mineral's surface reflects light, and it is the first fork in every identification key: metallic or nonmetallic. Streak tells you the color of the mineral's powder, which you see by rubbing it across a piece of unglazed porcelain. Both are more dependable than the color of the specimen, which traces of impurities can change completely. A black, metallic-looking hematite leaves a rust-red streak; brassy pyrite leaves a greenish-black one; gold leaves gold [1][2][3]. This page shows how to run a streak test without marring a specimen, lists the minerals whose streak surprises people, defines the luster terms with examples, and explains why color alone so often misleads.
Luster comes first
The Mineralogical Society of America's identification key tells collectors to determine luster first, calling it "the first critical step in keying out a mineral," and defines it as the overall sheen of the surface, which should not be confused with color [4]. Pyrite and galena look nothing alike in color and both have metallic luster [4].
The luster terms
The MSA key divides luster into metallic, submetallic and nonmetallic, and splits nonmetallic into six kinds [4]:
| Luster | What it looks like | Examples |
|---|---|---|
| Metallic | Opaque, shines like polished metal | Pyrite, galena [4] |
| Submetallic | Dull metal shine, often in dark minerals | Some sphalerite and cinnabar can look submetallic though their true luster is resinous or adamantine [4] |
| Adamantine | Brilliant, diamond-like | Cinnabar crystals [2] |
| Resinous | Like dried resin or amber | Sphalerite [5] |
| Glassy (vitreous) | Like broken glass | Quartz, fluorite, feldspar [3] |
| Pearly | Soft sheen like the inside of a shell, often on cleavage surfaces | Dolomite can be vitreous to pearly; alabaster gypsum earthy to pearly [3] |
| Silky | Sheen of fine parallel fibers | Fibrous minerals in fine parallel growth |
| Earthy | Dull, no shine, like dry soil | Earthy hematite, some calcite [3] |
Look at a fresh surface in good light and turn the piece. Tarnish and weathering change luster: the SUNY Potsdam tables note that chalcopyrite "may tarnish to dull brass or brown" [3]. If one face looks dull and a fresh break shines, the fresh break is the luster.
The streak test
Streak is the color of a mineral in powdered form. Grinding a crystal into powder removes the effects of crystal size, surface tarnish and the way light passes through a large piece, so the powder color is steadier than the color of the specimen.
What you need
An unglazed porcelain streak plate. The back of a white bathroom tile works if the back is unglazed. Keep a black plate too if you can: a white powder shows up better on black.
Running the test without harming the piece
- Find a corner or edge on the back of the specimen, or a loose fragment from the same piece. You need only a few grains of powder.
- Hold the plate flat on a table. Do not hold it in the air; the plate can snap.
- Draw the corner across the plate once, lightly, for a short distance. Look at the trail of powder.
- Blow or wipe away the coarse grains and look at the fine powder. That is the streak.
- Clean the plate with water and a scrubbing pad before the next test.
When there is no streak
A streak plate has a hardness of about 6.5 to 7 according to one university lab handout, and about 7.0 according to Britannica [6][7]. A mineral harder than the plate scratches it instead of leaving powder, so quartz, topaz, corundum and most garnets show no streak, or a white line of powdered porcelain. Most nonmetallic minerals leave a white or very pale streak, which is why the Mesa Community College lab says streak matters less for them [8]. Streak is at its most useful for metallic minerals and for strongly colored nonmetallic ones.
Minerals whose streak is not their color
| Mineral | Typical color | Streak | Source |
|---|---|---|---|
| Hematite | Steel-gray, black or dark brown when crystalline; red when earthy | Rust-red to Indian-red | [1][5] |
| Pyrite | Golden or brassy yellow | Greenish or brownish black | [3] |
| Chalcopyrite | Brass-yellow | Black (MSA); greenish-brown (SUNY Potsdam) | [2][3] |
| Gold | Golden yellow | Golden yellow | [2] |
| Galena | Lead-gray, silvery | Gray to lead-gray | [3][9] |
| Magnetite | Black to dark gray | Black | [3] |
| Goethite | Dark brown to black | Yellow-brown to ocher-yellow | [5] |
| Cinnabar | Dark red to vermilion | Dark red to bright red | [1][5] |
| Sphalerite | Dark to light brown, reddish brown, black | Light brown | [5] |
| Malachite | Dark to bright green | Light green | [5] |
| Azurite | Deep azure blue | Light blue | [5] |
| Fluorite | Green, purple, light blue and others | White | [3] |
The sources do not always agree on wording for the dark sulfides. For chalcopyrite, the MSA key gives "black," the SUNY Potsdam tables give "greenish-brown," and the Mineralogical Society of America's Mineralogy4Kids pages give "green-black" [2][3][9]. They describe the same dark streak with a green cast. This page records all three so you know any of them is a match.
Streak tests to skip
Cinnabar is a mercury mineral and galena a lead mineral. National Park Service guidance warns that cinnabar can release mercury vapor and that body heat can vaporize mercury from it, and lists lead and mercury among the toxic elements in collection minerals [10]. Powdering them on a plate makes dust you then wipe off with your fingers. If the locality and the other tests already point to one of these, you do not need the streak. If you do run it, wear gloves and wash up afterward; the full list of species to treat this way is on which minerals are dangerous.
Why color alone misleads
Many minerals take their color from small amounts of other elements or from defects in the crystal, not from their main ingredients. The SUNY Potsdam tables list quartz as colorless, white, purple, pink or gray, and fluorite as usually green or purple, sometimes light blue [3]. Sphalerite runs from yellow-brown to black [5]. Two different minerals can share a color (pyrite and chalcopyrite are both brassy) and one mineral can come in five. Color is still worth recording, because certain colors are strong hints (a bright blue that streaks light blue is a good start toward azurite), but no key relies on it alone.
Weathering changes color too. Iron staining turns pale minerals orange-brown, and copper minerals tarnish. Always judge color on a fresh surface, and use streak to check it.
Putting luster and streak together
In practice the two tests work as a pair. Decide luster, then streak, and you have usually cut the candidate list to a handful before you pick up a knife:
- Metallic luster, dark streak: pyrite, chalcopyrite, galena, magnetite and other sulfides and oxides. Hardness and specific gravity sort them.
- Metallic luster, colored streak: gold (yellow), copper (coppery red) and hematite (red) [2].
- Nonmetallic luster, colored streak: the copper carbonates, cinnabar, goethite, sphalerite and others [5].
- Nonmetallic luster, white streak: the large group that includes quartz, feldspars, calcite, fluorite and gypsum. Hardness and cleavage do the work here.
Testing hardness next
For the last group especially, the next test is the scratch test. The method, the ten Mohs minerals and the values for a fingernail, coin, knife and glass are on testing hardness with the Mohs scale, and the identification key takes luster, streak and hardness together.
References
- Mineralogical Society of America, The Mineral Identification Key, Table IA: Minerals with metallic or submetallic luster and hardness of less than 2½ (undated, read October 2026). https://msaweb.org/msa/Collectors_Corner/id/mineral_id_keytia.htm
- 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
- 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
- Mineralogical Society of America, The Mineral Identification Key, "Mineral Properties & Luster" (undated, read October 2026). https://msaweb.org/msa/Collectors_Corner/id/mineral_id_keyi3.htm
- 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
- University of Notre Dame, Planet Earth laboratory, Mohs hardness scale handout (undated, read October 2026). https://www3.nd.edu/~cneal/planetearth/Lab-Mineral-ID/MOH.pdf
- Encyclopaedia Britannica, "Mohs hardness," revised and updated by Erik Gregersen, published 10 May 2025. https://britannica.com/print/article/387714
- 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
- Mineralogical Society of America, Mineralogy4Kids, "Mineral Identification" (metallic minerals that do not scratch glass) (undated, read October 2026). https://min4kids.org/?p=358
- 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
