Mineralogy for beginners
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The scientific basics for understanding a stone: crystal systems, luster, hardness, density, inclusions, rocks and minerals, origin and responsible sourcing.
MineralogyThe cubic systemIn the cubic system, three equal axes meet at right angles. Pyrite, halite, fluorite, garnet and diamond belong here, but not all of them form cubes.MineralogyThe tetragonal systemIn the tetragonal system, two axes are equal and the third differs, all perpendicular. Zircon, rutile, vesuvianite, wulfenite and apophyllite illustrate it.MineralogyThe hexagonal systemThe hexagonal system has a single c axis and two equal axes at 120° in the basal plane. Beryl (emerald, aquamarine), apatite and graphite belong here; quartz is trigonal.MineralogyThe trigonal systemThe trigonal system is built around a three-fold symmetry axis. Quartz, calcite, corundum (ruby, sapphire), tourmaline and hematite belong to it, often with crystals mistaken for hexagonal ones.MineralogyThe orthorhombic systemIn the orthorhombic system, the three axes are perpendicular but all of different lengths. Topaz, peridot, alexandrite, aragonite, barite and sulphur are examples.MineralogyThe monoclinic systemIn the monoclinic system, only one angle differs from 90°: the cell is tilted in one direction. Gypsum, micas, talc, orthoclase, malachite, azurite, jade and epidote belong here.MineralogyThe triclinic systemThe triclinic system is the least symmetrical: three unequal axes and no right angles. Albite, labradorite, amazonite, turquoise, kyanite and rhodonite are examples.MineralogyWhat is a mineralA mineral is a natural, crystalline solid of defined chemical composition, formed by geological processes. Here is what makes one, and what does not qualify.MineralogyMineral or rockA mineral is a substance of defined composition; a rock is an assemblage of minerals (or of glass or organic matter). Granite, lapis lazuli, jade, jasper: where does each one belong?MineralogyLuster of mineralsLuster describes how the surface of a mineral reflects light: vitreous, metallic, adamantine, pearly, silky, greasy. It depends mainly on the refractive index.MineralogyCleavage and fractureCleavage is a crystal's tendency to split along smooth planes; fracture is breaking away from those planes. Mica, calcite, halite, quartz and obsidian show the different types.MineralogyDensity of a stoneDensity compares the mass of a stone with that of an equal volume of water. It helps tell quartz, glass, amber or zirconia apart, and you can measure it at home with a scale and a glass of water.MineralogyStreak of a mineralStreak is the colour of the fine powder left by a mineral rubbed on unglazed porcelain. It is more consistent than the colour of the crystal: grey hematite leaves a red-brown streak.MineralogyTransparency and translucencyA stone is transparent if you can see clearly through it, translucent if it lets light pass without a sharp image, and opaque if it blocks light. Cutting and price depend on it.MineralogyInclusionsAn inclusion is a foreign body (crystal, liquid, gas) or a fracture trapped inside a crystal. Rutile, chlorite, bubbles, fissures: each one tells the story of how the stone formed.MineralogyColour and pleochroismA mineral’s colour comes from its composition, from impurities, from defects or from optical effects. Pleochroism is the change of colour depending on the direction of observation.MineralogyLuminescenceSome minerals emit visible light when lit with UV: this is fluorescence. Calcite, fluorite, ruby, willemite: mechanism, examples and UV lamp safety.MineralogyIgneous rocksIgneous rocks form when magma or lava cools and solidifies. Slow cooling gives large crystals (granite); rapid cooling gives fine grains or glass (basalt, obsidian).MineralogySedimentary rocksSedimentary rocks form when sediments accumulate and harden: grains, shells, salts. Limestone, sandstone, chalk, gypsum, petrified wood, ammolite and stromatolite all come from them.MineralogyMetamorphic rocksMetamorphic rocks are rocks transformed by heat and pressure without complete melting: limestone becomes marble, sandstone becomes quartzite. Garnet, lapis lazuli and jade all come from this process.MineralogyGeodes and drusesA geode is a cavity lined with crystals that grew toward the centre; a druse is a carpet of crystals. Gas from lava, silica-laden waters, very long timescales: here is how they form.MineralogyPegmatitesPegmatites are igneous rocks with crystals over one centimeter across, sometimes several meters long. They host tourmaline, beryl, topaz, kunzite, lepidolite and lithium.MineralogyResponsible stone extractionWhere do stones come from, and under what conditions are they mined? Industrial and artisanal mining, documented risks, legal frameworks and labels: a factual overview.MineralogyTraceability and originMine, export country, cutting workshop, country of manufacture: the word “origin” covers several realities. Here is what can be traced, by whom, and the limits of proof.MineralogyStones and the environmentA stone has an impact at every stage: extraction, cutting, treatments, transport, packaging. Here is what is known, what is poorly measured, and the simple habits that limit the damage.MineralogyLabels and certificates: what are they worthLaboratory report, sector label, seller's certificate: three very different things. What each one attests, what it does not, and how to avoid false certificates.
The meanings mentioned belong to tradition and symbolism. They are neither promises of results nor medical advice.