Identifying a mineral is a short checklist. Measure what you can on the specimen, then open the species page. Color is last, not first. This page works three stones already filed here, quartz, fluorite, and obsidian, against public mineral URLs.

Four checks before you name a species

The Mohs workflow is the same order. Do this on the table, then open Mindat or the Handbook PDF.

  1. Streak. The color of the powder on unglazed porcelain. Hematite streaks red-brown even when the lump looks silver. Quartz and fluorite often streak white or leave no useful mark. That is still a result. USGS mineral pages treat streak as a field test, not a color name.
  2. Hardness. What it scratches and what scratches it, against the USGS Mohs sequence. Quartz at 7 versus fluorite at 4 is the useful split on this catalog. Work the two stones against each other before you invent a variety name.
  3. Cleavage. How it breaks. Fluorite cleaves octahedrally, smooth triangular faces after a knock. Quartz does not; it fractures. Glass (obsidian) shows conchoidal, shell-like curves. Cleavage is a species fact, not a chip from shipping.
  4. Habit. The shape the specimen actually grew: prism, cube, massive, or glass. See the bins below.

Then name the species. Variety (amethyst, citrine, rose) comes after SiO2 is honest.

Habit: prism, cube, massive, glass

Habit is the shape the specimen actually grew. Four bins cover most pieces people bring to a table:

  • Prism. The Handbook of Mineralogy quartz species PDF describes quartz as hexagonal, with prismatic crystals terminated by pyramidal faces. A six-sided point with a pyramidal tip is the usual quartz look, not a cube.
  • Cube. Webmineral lists fluorite in the isometric crystal system with the cube form (100). A square-faced block, often with octahedral corners, is fluorite habit, not quartz.
  • Massive. No crystal faces you can name. Rose quartz is commonly sold this way: pink quartz without the sharp points of a prism. That is still the quartz species, not a separate mineral.
  • Glass. USGS describes obsidian as volcanic glass that cooled so fast crystals did not have time to form. Conchoidal (curved, shell-like) fracture, often black, used for tools. It is not a crystal lattice.

If you cannot name the habit, do not skip to color. Habit plus hardness already splits quartz from fluorite from glass.

Mohs checklist: quartz 7 vs fluorite 4

Webmineral lists quartz hardness as 7 and fluorite hardness as 4. The Handbook of Mineralogy quartz PDF records hardness 7 (variable by direction and form). That three-step gap is the most useful check on this hub.

SpecimenSpecies pageMohsWhat it does on the table
Quartz SiO2 7 Scratches fluorite. Marks window glass. Does not take a knife scratch.
Fluorite CaF2 4 Will not scratch quartz. A knife or a quartz point will mark it. Keep it as a display stone, not a pocket stone.
Obsidian volcanic glass Not a Mohs mineral species. USGS: glass, conchoidal fracture, historically knapped for tools.

Work the two minerals against each other before you name a color. Quartz scratches fluorite; fluorite does not scratch quartz. That result is the species split, not a variety name.

Why color comes last

The quartz species PDF lists colorless, white, rose-pink, yellow, green, blue, violet, brown, and black. One formula, many colors. Webmineral files those colors as varieties of quartz, not as separate IMA species: amethyst (purple), citrine (yellow), rose quartz (rose-colored).

Color also lies in the trade. GIA’s citrine FAQ states that yellow quartz is rare in nature and that most citrine is the result of heating, which converts purple amethyst to the golden shades sold as citrine. That is a treatment fact, not a species split: heated amethyst is still SiO2. Do not name a yellow point “citrine” from color alone, and do not treat citrine as a different mineral from quartz.

Glass that people file with crystals

USGS Yellowstone pages describe obsidian as volcanic glass. The tool-making lava-flow note records conchoidal fracture and knapping for knives and points. A later rhyolite anatomy note states that obsidian is glass that cools so rapidly crystals do not have time to form. File obsidian with the stones people group as crystals if you want; cite it as glass, not as a crystal lattice, and not as studio glass.

A working order

  1. Habit: prism, cube, massive, or glass.
  2. Hardness: quartz at 7 vs fluorite at 4. Use the two stones, or a known quartz point, before you invent a name.
  3. Color last. It misleads. Heated amethyst sold as citrine is a trade fact, not a species split.
  4. Open the species PDF or Webmineral page and match cleavage, luster, and formula. Fluorite has perfect {111} cleavage (Webmineral). Quartz cleavage is indistinct; fracture is conchoidal.

Worked examples

1. Clear six-sided point. Habit is a hexagonal prism with a pyramidal tip. A steel knife does not scratch it; it scratches a fluorite cube. Color is water-clear, so color tells you nothing. Handbook of Mineralogy + Webmineral: quartz, SiO2, Mohs 7. Open the quartz PDF.

2. Purple cube. Habit is cubic. A quartz point scratches it. Perfect octahedral cleavage. Purple is common on fluorite and on amethyst. Color cannot finish the ID. Webmineral: fluorite, CaF2, Mohs 4, isometric. USGS fluorspar pages cover the industrial mineral, not a specimen price.

3. Black glassy flake with a curved break. No prism, no cube. Conchoidal fracture, sharp edge. USGS Yellowstone: obsidian, volcanic glass, historically used for tools. It is not quartz, not fluorite, and not a crystal species.

4. Yellow “citrine” cluster. Habit is a quartz druse (prismatic tips). Hardness 7. Color is golden. GIA: most citrine on the market is heated amethyst. Name the species (quartz) first. Variety (citrine vs heated amethyst) is a trade label on the same SiO2 host. See Quartz varieties.

Collecting and display are uses. Jewelry is a use some people make of a stone. This hub stays on the mineral fact. Cards under Kinds name the host and the URL.

Tools for this checklist vs tools for a shelf: ID kit vs display stand. The identification cluster is a jeweler loupe plus a Mohs kit, affiliate suggestion, not our stock: Identification kit.

Sources

  1. Anthony, J.W., Bideaux, R.A., Bladh, K.W., and Nichols, M.C., Handbook of Mineralogy, Mineralogical Society of America. Quartz (SiO2) species PDF: https://www.handbookofmineralogy.org/pdfs/quartz.pdf
  2. Handbook of Mineralogy. Fluorite (CaF2) species PDF: https://www.handbookofmineralogy.org/pdfs/fluorite.pdf
  3. Webmineral. Quartz mineral data (formula SiO2, hardness 7, varieties including amethyst, citrine, rose quartz): https://webmineral.com/data/Quartz.shtml
  4. Webmineral. Fluorite mineral data (formula CaF2, hardness 4, isometric, cube form, {111} cleavage): https://webmineral.com/data/Fluorite.shtml
  5. U.S. Geological Survey, National Minerals Information Center. Silica statistics and information: https://www.usgs.gov/centers/national-minerals-information-center/silica-statistics-and-information
  6. U.S. Geological Survey, National Minerals Information Center. Fluorspar statistics and information: https://www.usgs.gov/centers/national-minerals-information-center/fluorspar-statistics-and-information
  7. U.S. Geological Survey, Yellowstone Volcano Observatory. Yellowstone’s tool-making lava flows (obsidian, conchoidal fracture, knapping): https://www.usgs.gov/observatories/yvo/news/yellowstones-tool-making-lava-flows
  8. U.S. Geological Survey, Yellowstone Volcano Observatory. Rock, glass, and flowbands: Yellowstone’s rhyolite anatomy (obsidian as volcanic glass): https://www.usgs.gov/observatories/yvo/news/rock-glass-and-flowbands-yellowstones-rhyolite-anatomy
  9. Gemological Institute of America. “Is citrine treated?” (most citrine is heated amethyst): https://www.gia.edu/gia-faq-treated-citrine

Updated 2026-08-31.