Mineralogy for the Amateur: Crystals, Hardness, and Where to Find Specimens
Mineralogy does not require a laboratory. A hand lens, a streak plate, a reference field guide, and a good collection locality will teach you more in a day than a month of reading. The science of minerals is built on systematic observation, and systematic observation is a skill any patient visitor to a mine or a museum can practise and improve.
The Mohs Hardness Scale
Friedrich Mohs published his relative hardness scale in 1812. Its ten reference minerals remain the standard for field identification: talc (1), gypsum (2), calcite (3), fluorite (4), apatite (5), orthoclase feldspar (6), quartz (7), topaz (8), corundum (9), and diamond (10). Each mineral will scratch all those below it. Your fingernail is approximately 2.5, a copper coin about 3.5, a steel knife blade 5.5, and a piece of glass 5.5. These are your field reference tools. A mineral that a steel blade cannot scratch is almost certainly quartz or harder; one that your nail scratches is gypsum or calcite. Hardness alone will not identify a mineral but it quickly eliminates most possibilities.
The Seven Crystal Systems
All minerals that crystallise belong to one of seven systems, defined by the symmetry of the unit cell. Cubic (isometric): four three-fold rotation axes, producing cubes, octahedra, and dodecahedra — galena, pyrite, halite, fluorite. Tetragonal: one four-fold axis — zircon, cassiterite. Hexagonal: one six-fold axis — quartz, calcite, beryl, apatite. Trigonal (sometimes classified within hexagonal): three-fold axis — tourmaline, rhodochrosite. Orthorhombic: three two-fold axes, all at right angles but unequal lengths — topaz, baryte, celestine. Monoclinic: one two-fold axis — gypsum, orthoclase, malachite, azurite. Triclinic: no rotation axes above one-fold — plagioclase feldspars, kyanite, rhodonite. Identifying crystal form is one of the most satisfying moments in mineral identification.
Cleavage and Fracture
Cleavage is the tendency of a mineral to break along flat planes parallel to crystallographic faces — an expression of weaker bonding in those directions. It is described by number of directions and angle: calcite has three cleavage directions meeting at non-right angles (rhombohedral), galena has three meeting at right angles (cubic), muscovite has one perfect basal cleavage. Fracture is what happens where cleavage does not: conchoidal fracture (smooth, curved, like broken glass — typical of quartz and obsidian), irregular fracture, splintery fracture, and hackly fracture (jagged metal-like, common in native copper). Distinguishing cleavage from fracture under a hand lens is a core skill: cleavage surfaces will catch the light evenly across a flat plane; fracture surfaces will be irregular.
The Smithsonian and the Natural History Museum, London
The Janet Annenberg Hooker Hall of Geology, Gems, and Minerals at the Smithsonian National Museum of Natural History in Washington DC houses the Hope Diamond (45.52 carats, blue IIb diamond, formerly property of Louis XIV), the Carmen Lúcia Ruby (23.1 carats, from Mogok, Burma), and one of the largest gem and mineral collections open to the public anywhere. The mineral diversity specimens — perfect fluorite octahedra from Rogerley, England; self-healed quartz from Arkansas; proustite from Chañarcillo, Chile — are as instructive as the gem cases.
The Natural History Museum in London holds one of the oldest systematic mineral collections in the world, founded on the collections of Sir Hans Sloane (1753) and subsequently expanded. The Vault gallery houses meteorites, large crystal specimens, and the Aurora Pyramid of Hope — a display of 296 naturally coloured diamonds. The mineral galleries are free. A serious amateur should visit both institutions at least once.
Where to Find Minerals Yourself
Herkimer Diamonds, New York
Herkimer County in upstate New York produces doubly-terminated quartz crystals in dolostone, known as Herkimer Diamonds, of unusually high clarity. They occur in pockets in the Little Falls Dolostone, a Cambrian-age carbonate formation. Several pay-to-dig sites operate in Herkimer and adjacent Montgomery County; the most established is Herkimer Diamond Mines in Middleville. No special equipment is needed beyond a rock hammer and eye protection; the crystals are found by carefully splitting the dolostone along bedding planes.
Bancroft, Ontario
The Bancroft area in eastern Ontario sits on the Grenville Province, a Precambrian terrane that has produced over 150 mineral species. Fluorapatite, massive sodalite, rose quartz, amazonite, magnetite in large crystals, and titanite are among the accessible targets. The Bancroft Gem and Mineral Show in August is one of the largest in eastern North America. The Gemboree Mineral Collecting field trips give permitted access to working localities.
Cornwall, UK: Stannite and the Mine Dumps
Cornwall produced tin and copper in quantities that made it the centre of world non-ferrous production before 1870. The mine dumps of the Camborne-Redruth district and the St Just area yield cassiterite, wolframite, arsenopyrite, chalcopyrite, and secondary minerals including malachite, azurite, linarite, and turquoise. Geevor Tin Mine (Pendeen) has designated collecting areas on its surface dumps for paid visitors. Stannite — copper-iron-tin sulphide — is specifically a Cornish mineral, first described from here in 1797.
The Polariscope and Optical Mineralogy
A polariscope (or polarising microscope) passes light through a polarising filter, through a thin section of mineral or gemstone, and through a second filter rotated 90 degrees. Under this arrangement, isotropic minerals (cubic system) remain dark when rotated; anisotropic minerals produce interference colours that change with rotation. The Michel-Lévy interference colour chart maps these colours to birefringence and thickness. For gems, a simple handheld dichroscope — which splits light into two polarised beams — shows pleochroism (different colours in different crystal directions), useful for identifying tourmaline, tanzanite, iolite, and ruby. Neither instrument requires laboratory training; both reward patient practice.
Building a Reference Collection
Start with the rock-forming and ore minerals most likely to appear at sites you visit: quartz in multiple habits, calcite, pyrite, galena, chalcopyrite, malachite, fluorite, baryte, hematite, magnetite, and gypsum. Buy labelled specimens from reputable dealers, verify them against the properties in a systematic reference such as the Mineralogical Record or Dana's System of Mineralogy, and label each with locality, date, and source. A reference collection of fifty well-documented specimens will teach you more than five hundred randomly acquired ones.
Streak and Lustre: Two More Diagnostic Tools
Streak — the colour of a mineral's powder produced by dragging it across an unglazed porcelain streak plate — is often more diagnostic than surface colour, because surface colour is easily affected by trace impurities or oxidation. Hematite, for example, can be metallic black, red, or grey on its surface, but it always leaves a red-brown streak. Pyrite is brass-yellow on the surface but leaves a greenish-black streak. Magnetite: black streak. Gold: yellow streak (one of the ways to distinguish gold from pyrite).
Lustre describes the quality of light reflected from a mineral surface and is described in a standard series: metallic (mirror-like, opaque minerals), submetallic, adamantine (diamond-like, very high reflectance), vitreous (glass-like, the most common in silicates), resinous, waxy, greasy, silky (fibrous minerals like satin spar gypsum), pearly (micas and carbonates on cleavage faces), and earthy (dull, clay-like). A combination of hardness, streak, cleavage, crystal system, and lustre will identify most common minerals without a laboratory.
Using the Map for Mineralogical Travel
Mining districts are mineralogical districts. The lead-zinc skarn deposits of the English Pennines (Killhope, Allenheads), the copper porphyries of the American southwest, the skarn iron deposits of central Sweden, and the pegmatite belts of Madagascar, Brazil, and California are all accessible to amateur visitors at specific sites. The interactive map plots heritage mining sites by commodity: filter by tin, copper, lead-zinc, or silver to find the districts most likely to reward a collecting-oriented visit. Always confirm collecting permissions before picking up specimens — many show mines prohibit collecting, and taking specimens from a UNESCO site is illegal.