Mineral Identification Chart: Hardness, Streak, Luster

Work down the chart in order and stop when the answer is forced. The properties most commonly used to identify minerals are colour, luster, hardness, streak, cleavage or fracture and crystal habit, and they are not equally useful — colour is the weakest and hardness is usually the strongest.

The order that wastes the least effort:

• Luster first, because it is free and it splits the whole set in two.
• Hardness second, because it is the single most decisive number.
• Streak third, especially for anything with a metallic look.
• Cleavage or fracture fourth, using a freshly broken edge.
• Heft and any special property last, to confirm.

Write each result down as you go. Three recorded properties usually narrow a specimen to a handful of candidates; guessing from memory rarely does.

Hardness is resistance to scratching, described against the ten reference minerals of the Mohs scale. The U.S. Geological Survey gives them as:

• Talc 1
• Gypsum 2
• Calcite 3
• Fluorite 4
• Apatite 5
• Feldspar 6
• Quartz 7
• Topaz 8
• Sapphire 9
• Diamond 10

Everyday objects used in teaching labs fill in between them:

• 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

The knife blade at 5.5 is the practical divide between hard and soft minerals. Test on a hidden edge, draw the tool once with firm pressure, then wipe and look with a lens: smeared metal wipes away, a true scratch is a groove you can feel.

Streak is the colour of a mineral's powder, obtained by dragging the specimen across an unglazed white porcelain plate. It is more diagnostic than surface colour because weathering, impurities and coatings change how a mineral looks without changing its powder.

The entries worth memorising:

• Hematite gives a reddish-brown streak no matter what the specimen looks like — silvery, black, red or earthy.
• Pyrite gives black to greenish-black streak despite its brassy yellow surface, which is the classic check against gold.
• Anything harder than the plate leaves no powder and scratches the plate instead. Quartz at 7 behaves this way, and that result tells you the mineral is hard even though you got no colour.

For a hard mineral you can still get a streak by crushing a small chip to powder with a hammer, wearing eye protection, and looking at the colour of the powder itself.

Luster describes how the surface reflects light. The first split is metallic — looking like chrome, steel, silver or gold — versus non-metallic. Within non-metallic, the classes you will use are vitreous or glassy, earthy and dull like dried mud, silky with the soft sheen of fibres, pearly like the inside of a shell, and submetallic for a dull metal look.

Cleavage is a tendency to break along smooth, flat, parallel planes that reflect the atomic structure. Record three things: quality (perfect, imperfect or none), the number of directions, and the angles between them. Mica has one perfect cleavage and splits into sheets; calcite breaks into rhomb-shaped blocks.

Fracture is what you get without cleavage. Conchoidal fracture is the curved, shell-like surface seen on quartz, olivine and volcanic glass; other minerals fracture irregularly, splintery or fibrous.

One warning about crystal faces: a flat, shiny face is not automatically cleavage. A grown crystal face and a cleavage surface can look alike, but cleavage repeats as parallel steps through the specimen while a grown face does not.

Specific gravity is the weight of a specimen relative to an equal volume of water. Few people measure it in the field; experienced geologists judge it as heft, a subjective sense of how heavy a piece feels for its size. Comparing two specimens of similar size in your two hands is the practical version.

Magnetism identifies a narrow group: some iron-oxide minerals attract a magnet, magnetite strongly and ilmenite weakly.

The acid test distinguishes carbonates. A drop of dilute hydrochloric acid, about 10 to 15%, effervesces on calcite; dolomite fizzes only after it has been powdered. This is a laboratory procedure. Dilute hydrochloric acid injures skin and eyes, needs eye protection, gloves and ventilation, and it etches or destroys the specimen surface. If you only need to know whether something is calcite, a copper coin at 3.5 will scratch it and answer the question safely.

A chart is only useful if it travels with you. The field kit that covers everything above fits in a jacket pocket: a copper coin, a steel nail, a knife or a glass shard, a square of unglazed tile, a small magnet, a hand lens and a notebook.

Record results as a short line per specimen — luster, hardness bracket, streak colour, break pattern, heft — rather than a conclusion. A note reading "non-metallic, vitreous, harder than a nail, no streak, conchoidal" stays useful forever, while a note reading "quartz?" does not.

Photograph each specimen against a plain background with a coin for scale. Photograph it and the app identifies it, then check the suggestion against your recorded properties before you accept it.

Mineral identification works best as a key rather than a guess. The Mineralogical Society of America's Mineral Identification Key, written by Alan Plante, Donald Peck and David Von Bargen with Mindat.org, works like a wildflower key: a few diagnostic properties send you to a table of likely suspects, and a guide book or database confirms the final name. Its practical advice:

• Test a fresh, unweathered surface, on an inconspicuous spot rather than a good crystal face.
• Test hardness both ways: scratch the tool with the mineral, and the mineral with the tool.
• Repeat unclear tests. Several streaks and several hardness tries beat one.
• Check cleavage on breaks you made yourself, with a 10x lens, not on surfaces nature broke.
• If it does not key out, start again from the top, rethinking calls such as whether the luster is really metallic.

The key covers only a couple of hundred of the most commonly seen species, and it warns that a sample that looks rare is usually a common mineral in an unusual form. Sending a sample to a lab should be the last step.

The MSA key's identification kit adds detail the usual household lists leave out:

• Copper: a penny from before 1983, or a small piece of copper or heavy copper wire.
• Knife: a cheap pocketknife, not a Swiss Army knife, whose harder steel throws hardness readings off.
• Steel file: a 2 to 3 inch tip of a triangular chainsaw file.
• Reference minerals: small pieces of fluorite, quartz, beryl or topaz, and corundum with sharp points, to test the upper steps of the scale.
• Streak plate: an unglazed porcelain tile, a 2-inch square is plenty.
• Magnet, 10x loupe, tweezers and labels.

It also lists a candle for fusibility and dilute 10% hydrochloric acid for carbonates. Both need care; the acid in particular belongs in a supervised setting.

The U.S. Geological Survey says minerals must be examined in person from all sides and are extremely difficult to identify from photographs. It points people to their state geological survey, natural science museums, university geology departments, rock shops, local gem and mineral or rockhounding clubs, and vendors at gem and mineral shows. Bring your recorded test results; they make the conversation much shorter.