Crystal Identification: Tests That Name a Crystal

The fastest way to misidentify a crystal is to start from its colour. Introductory geology courses make this point early: mineral colours are affected both by the main elements and by impurities, so the same element can produce different colours in different minerals, and one mineral can appear in several colours.

Quartz is the clearest example. Colourless rock crystal, milky white, smoky brown, purple amethyst and pink rose quartz are all the same mineral with the same hardness and the same break pattern, and the U.S. Geological Survey lists them together as coarsely crystalline quartz varieties.

What works instead is a short sequence of physical tests, run in the same order every time: luster, hardness, streak, cleavage or fracture, then heft and any special property. Four of those tests need nothing but a coin, a nail, a piece of unglazed tile and good light.

Hardness is a mineral's resistance to scratching, measured against the ten reference minerals of the Mohs scale, which the U.S. Geological Survey gives as talc 1, gypsum 2, calcite 3, fluorite 4, apatite 5, feldspar 6, quartz 7, topaz 8, sapphire 9 and diamond 10.

Everyday objects fill the gaps, and these are the values used in teaching labs:

• Fingernail, about 2.5
• Copper coin, about 3.5
• Knife blade or window glass, about 5.5
• Steel nail, about 6.5
• Masonry drill bit, about 8.5

A steel pocketknife at 5.5 is the dividing line between hard and soft minerals, and knowing which side of it your crystal falls on already eliminates most of the possibilities.

Test on an inconspicuous edge: draw the tool once with firm pressure, wipe the spot and look with a lens. Smeared metal rubs off; a real scratch is a groove. Test both ways, mineral against tool and tool against mineral.

Streak is the colour of a mineral's powder, obtained by scratching it on an unglazed white porcelain plate. It is far more consistent than outside colour, because weathering, impurities and coatings change appearance but not the powder. Hematite always gives a reddish-brown streak, and pyrite gives black to greenish-black despite being brassy yellow. One catch: minerals harder than the plate scratch it rather than leave powder, so quartz at 7 will not streak.

Luster is how the surface reflects light, and it splits first into metallic — looking like chrome, steel, silver or gold — and non-metallic. The non-metallic classes worth learning are vitreous (glassy, like quartz), earthy or dull (like dried mud), silky (soft sheen, fibrous minerals), pearly (like the inside of a shell) and submetallic (dull metal).

Cleavage or fracture is often the deciding test. Cleavage means the crystal breaks along smooth, flat, parallel planes, and it is described by quality, by the number of directions and by the angles between them; mica splits along one perfect cleavage into sheets, and calcite into rhomb-shaped blocks. Fracture is what happens when there is no cleavage: quartz and olivine break along a curved, shell-like conchoidal surface.

Run the ladder and most of the common ones separate immediately.

• Clear quartz, amethyst, citrine, smoky and rose quartz. All quartz, all 7 on the Mohs scale, all breaking with a curved conchoidal surface and no flat cleavage. They will not be scratched by a steel nail and they scratch a streak plate.
• Calcite. Hardness 3, so a copper coin marks it easily, and it splits into rhombs — leaning blocks rather than cubes.
• Fluorite. Hardness 4, above a copper coin and well below a knife, and it breaks along flat octahedral planes, which is what gives cleaved fluorite its triangular faces.
• Selenite, the clear bladed form of gypsum. Hardness 2 — a fingernail at 2.5 scratches it. That single test separates selenite from every quartz variety instantly.
• Feldspar. Hardness 6: a steel nail at 6.5 scratches it, a knife at 5.5 does not.

Two cautions worth carrying. Citrine is usually not what it seems: GIA states that most citrine on the market results from heat treatment of amethyst, so a yellow quartz point that passes every physical test can still be treated material. And obsidian is not a crystal at all — it is volcanic glass, which is why it breaks conchoidally and shows no crystal faces.

Heft is a collector's shortcut for specific gravity, the weight of a specimen compared with an equal volume of water. Comparing two similar-sized specimens in your two hands tells you more than weighing one.

Magnetism settles a small group quickly: some iron-oxide minerals attract a magnet, magnetite strongly and ilmenite weakly.

The acid test is the classic one for carbonates. Geology labs put a drop of dilute hydrochloric acid, around 10 to 15%, on a specimen: calcite effervesces, while dolomite fizzes only once powdered. That belongs in a lab. Dilute hydrochloric acid burns skin and eyes, needs eye protection, gloves and ventilation, and it destroys the polish and crystals on a carbonate specimen. If you only want to know whether a crystal is calcite, the copper-coin hardness test answers it without chemistry.

Photograph the crystal in daylight against a plain background, with a coin for scale, and take a second frame of a broken edge, where fracture and cleavage read more clearly than on a polished face.

Photograph it and the app identifies it. Confirm with hardness and break pattern before you label the piece, because those are the properties a photograph cannot show.

A crystal is a single mineral with an orderly internal structure, which shows outside as flat faces meeting at fixed angles; a rock is an aggregate of one or more minerals, as the U.S. Geological Survey defines them. So look for:

• Flat, shiny faces and sharp edges repeating in the same shape: a crystal.
• One uniform material throughout, with the same luster and break everywhere: a mineral, even if it has no crystal faces.
• Grains of different colours, layers, pebbles or bubbles: a rock. Granite, for example, is quartz, feldspar and mica together.

Tumbled stones sit in between. They are usually a single mineral such as quartz or agate, so the tests on this page still work on them.

• Bubbles. IdentifyRock's crystal guide notes that glass usually contains perfectly round air bubbles, while natural crystals contain tiny cracks, other minerals or fluid-filled cavities. Look with a 10x lens.
• Hardness. Glass sits around 5.5; quartz is 7. Real quartz scratches a glass plate and a steel nail will not mark it.
• Shape. Natural quartz points have six sides. Moulded glass often has soft edges and seams.
• Colour. Electric blue or hot pink "crystal" geodes are dyed, according to Wikipedia's geode entry, because those colours do not form naturally.

The cool-to-the-touch test some sellers suggest is unreliable on small pieces; hardness and a lens are the tests that settle it.

• The MSA Mineral Identification Key, free online from the Mineralogical Society of America, walks from luster, hardness and streak to a short list of common minerals, then points you to Mindat to confirm.
• A person with the specimen in hand: the USGS recommends state geological surveys, natural science museums, university geology departments, rock shops, gem and mineral clubs and show vendors, and warns that minerals are extremely difficult to identify from photographs alone.
• A photo app as a first step. RockLens names the likely mineral with a confidence level and alternatives; the tests above confirm it. The guides on this site are free.