Reference
Crystal Identification Guide
A step-by-step system for identifying any crystal or mineral using simple tests you can do at home. No expensive equipment required.
Follow these six tests in order. Each step narrows down the possibilities until you reach an identification.
Test the Hardness
How hard is it?
Hardness immediately eliminates most possibilities. A mineral that scratches glass cannot be calcite, fluorite, or any of the soft minerals. A mineral scratched by a fingernail cannot be quartz.
How to test: Try scratching the specimen with your fingernail (2.5), a copper coin (3.5), a steel knife (5.5), and a piece of quartz (7). Always test on an inconspicuous area. The hardest object that fails to scratch it tells you the minimum hardness.
Fingernail scratches it
Hardness below 2.5. Think talc, gypsum, selenite, soapstone, gold, silver.
Coin scratches it, fingernail does not
Hardness 2.5-3.5. Think calcite, mica, pearl, copper minerals.
Knife scratches it, coin does not
Hardness 3.5-5.5. Think fluorite, apatite, lapis lazuli, turquoise.
Quartz scratches it, knife does not
Hardness 5.5-7. Think feldspar, opal, pyrite, hematite.
Nothing scratches it
Hardness 7+. Think quartz family, topaz, corundum (ruby/sapphire), diamond.
Check the Luster
How does light reflect off the surface?
Luster narrows down mineral groups quickly. Metallic luster points you toward sulfides and native metals. Vitreous (glassy) luster covers most silicates. Waxy or earthy luster suggests weathered or microcrystalline minerals.
How to test: Look at the mineral on a fresh surface (not a weathered face) in good light. Compare the reflection quality to known references. Does it look like glass? Like metal? Like a wax candle?
Metallic (looks like metal)
Pyrite, galena, hematite, magnetite, gold, silver, chalcopyrite, marcasite.
Vitreous (looks like glass)
Quartz, feldspar, topaz, garnet, tourmaline, beryl, olivine. Most common luster.
Adamantine (brilliant, diamond-like)
Diamond, zircon, some sphalerite, cerussite. High refractive index minerals.
Waxy
Turquoise, serpentine, some chalcedony, chrysoprase. Microcrystalline or massive habit.
Pearly or silky
Mica, gypsum, satin spar selenite, tiger's eye, malachite (silky banding).
Earthy (dull, like clay)
Weathered specimens, limonite, bauxite, some hematite. Often indicates alteration.
Do the Streak Test
What color is the powder?
Streak is more reliable than surface color for identification. Many minerals come in multiple colors but always have the same streak. Hematite can look silver, black, or red, but always streaks reddish-brown.
How to test: Rub the mineral firmly across an unglazed white porcelain tile (a streak plate). The powder left behind is the streak. If the mineral is harder than 7, it will scratch the plate instead of leaving powder, so streak is not useful for very hard minerals.
White or colorless
Most silicates and carbonates. Quartz, feldspar, calcite, fluorite, topaz.
Reddish-brown
Hematite (the classic example). Also some iron-bearing minerals.
Greenish-black
Pyrite. This is the definitive test: pyrite looks gold but streaks dark.
Yellow to brown
Limonite, goethite, some sulfur.
Black
Magnetite, ilmenite, some hornblende.
Blue
Azurite, lazurite (lapis lazuli component).
Green
Malachite, some copper minerals.
Examine the Crystal Shape
What shape are the crystals?
Crystal habit (the typical form a mineral grows in) is a direct expression of its internal atomic structure. Recognizing habits speeds up identification dramatically. You will start to see a six-sided prism and immediately think quartz.
How to test: Use a 10x loupe to examine crystal faces, if visible. Note whether the crystal is prismatic (elongated), tabular (flat), equant (blocky), acicular (needle-like), or massive (no visible crystals). Count the number of faces and look for symmetry.
Six-sided prism with pointed tip
Almost certainly quartz or one of its varieties (amethyst, citrine, smoky quartz).
Perfect cubes
Pyrite, fluorite, galena, halite. All belong to the isometric system.
Octahedrons (8 faces)
Diamond, fluorite, spinel, magnetite.
Rhombohedrons (tilted cubes)
Calcite, dolomite, rhodochrosite. Trigonal system.
Rounded triangular prisms
Tourmaline. The cross-section is a distinctive rounded triangle.
Blade-like or flat
Kyanite, barite, gypsum. Monoclinic or triclinic systems.
Massive (no visible crystal faces)
Many minerals occur in massive form. You will need other tests to narrow down.
Check Cleavage and Fracture
How does it break?
Cleavage (breaking along flat planes) is controlled by crystal structure and is diagnostic for many minerals. Fracture (irregular breakage) patterns are also useful. Conchoidal fracture is distinctive of quartz and glass.
How to test: Look at broken surfaces. Are they flat and reflective (cleavage) or irregular (fracture)? How many directions of cleavage are there? At what angles do they meet? Do not break valuable specimens for this test. Examine existing broken surfaces.
Perfect cleavage in one direction (sheets)
Mica (muscovite, biotite), graphite. Peels into thin flakes.
Cleavage in two directions at ~90 degrees
Feldspar, pyroxene. Feldspars are the most common minerals on Earth.
Cleavage in two directions at ~60/120 degrees
Amphibole (hornblende). This angle distinguishes amphibole from pyroxene.
Cleavage in three directions at 90 degrees
Halite, galena. Breaks into cubes.
Cleavage in three directions, not at 90 degrees
Calcite. Breaks into rhombohedrons (tilted parallelograms).
Four directions of cleavage (octahedral)
Fluorite, diamond. Breaks into triangular fragments.
Conchoidal fracture (curved, shell-like)
Quartz, obsidian, flint. No cleavage at all.
Additional Tests
Still not sure? Try these.
Some minerals have unique diagnostic properties that make identification instant. A magnet, a bottle of vinegar, and a UV light can solve cases that hardness and luster cannot.
How to test: Apply the relevant test from the list below. These are non-destructive or minimally destructive tests that can be done at home.
Magnetic (attracted to a magnet)
Magnetite (strongly), some hematite, pyrrhotite. Very few minerals are magnetic.
Fizzes in vinegar or dilute acid
Calcite and aragonite (strongly), dolomite (weakly, with warm acid). The acid test for carbonates.
Fluorescent under UV light
Fluorite (often blue/purple), calcite (red/orange), willemite (green), scheelite (blue-white).
Tastes salty
Halite (rock salt). The only common mineral you should taste-test. Do not taste unknown minerals.
Feels greasy or soapy
Talc, serpentine, soapstone. The greasy feel is diagnostic.
Double image when you look through it
Calcite (optical grade). Double refraction is unique to calcite in easily visible form.
Unusually heavy for its size
High specific gravity. Galena (~7.5), barite (~4.5), hematite (~5.3). Compare to quartz (~2.65).
Commonly Confused Minerals
These pairs trip up beginners and even experienced collectors. Here is how to tell them apart with confidence.
Quartz vs. Glass
Quartz is harder than glass (7 vs. 5.5). Quartz will scratch glass. Glass often has small bubbles visible under 10x magnification. Quartz has conchoidal fracture with a waxy feel on broken surfaces.
Gold vs. Pyrite
The streak test settles it instantly. Gold streaks gold/yellow. Pyrite streaks greenish-black. Gold is also much softer (2.5 vs 6-6.5), much heavier, and malleable (bends, does not shatter).
Diamond vs. Quartz vs. Glass
Diamond scratches everything and nothing scratches it. It has adamantine luster and exceptional brilliance. Quartz has vitreous luster and is noticeably less brilliant. Glass is softer than both.
Calcite vs. Quartz
The acid test works perfectly. Calcite fizzes in vinegar, quartz does not. Calcite is softer (3 vs 7). Calcite has rhombohedral cleavage, quartz has conchoidal fracture.
Hematite vs. Magnetite
Both are iron oxides, both are dark and heavy. The magnet test works: magnetite is magnetic, hematite usually is not (some varieties are weakly magnetic). Hematite has a reddish-brown streak, magnetite has a black streak.
Turquoise vs. Dyed Howlite
Scratch an inconspicuous spot. Dyed howlite shows white underneath. Real turquoise is colored throughout. Howlite is also softer (3.5 vs 5-6). Under magnification, dye concentrates in surface cracks.
Quick Decision Flowchart
Does it have metallic luster?
Yes, and it is gold-colored: Test streak. Gold streak = gold. Black/green streak = pyrite or chalcopyrite.
Yes, and it is silver-colored: Test magnetism. Magnetic = magnetite. Heavy + cubic cleavage = galena. Light + flaky = mica.
Yes, and it is black: Test streak. Red-brown streak = hematite. Black streak = magnetite or ilmenite.
Does it have vitreous (glassy) luster?
Scratches glass (H 6+): Check crystal shape. Six-sided = quartz. Rounded triangular = tourmaline. Blocky/equant = feldspar or garnet.
Does not scratch glass (H below 5.5): Fizzes in acid = calcite. Cleavage in 4 directions = fluorite. Tabular/bladed = barite or gypsum.
Does it have waxy, earthy, or dull luster?
Blue-green: Turquoise (H 5-6), chrysocolla (H 2-4), or dyed howlite. Test hardness to distinguish.
Green banding: Malachite (H 3.5-4). Fizzes in acid. Always green streak.
Red-brown earthy: Likely limonite, laterite, or weathered hematite.
Feels soapy/greasy: Talc (H 1), serpentine (H 3-4), or soapstone.
Recording Your Observations
When identifying an unknown specimen, record these observations systematically. Having all the data in front of you makes identification much easier than trying to hold it all in your head.
Physical Properties
Color (surface and interior)
Streak color
Luster type
Hardness (tested against known objects)
Cleavage or fracture type
Transparency
Estimated weight / density feel
Crystal Properties
Crystal shape (if visible)
Number of crystal faces
Crystal system (if determinable)
Massive or crystalline habit
Special properties (magnetism, fluorescence, acid reaction)
Where it was found (locality, rock type)