Mineralogy explained: the main fields of mineral science

For the International Mineralogical Association (IMA), a mineral is an element or chemical compound, normally crystalline, formed by geological processes. Three ideas matter: a defined chemical composition, an ordered atomic structure, and a natural, geological origin.

This definition rules out several materials common in jewellery. Obsidian is a volcanic glass: its atoms are not ordered. Opal is silica without a true crystal structure: it is called a mineraloid. Pearls and mother-of-pearl are made by a mollusc: they are organic. Murano glass is man-made.

Finally, many stones are rocks, that is, mixtures of minerals. Lapis lazuli combines lazurite, calcite and pyrite. Unakite mixes epidote, pink feldspar and quartz. “Picasso jasper” is a marble. This is not a flaw: the mix is what draws their patterns.

The first steps are simple: look at colour, lustre and crystal shape, test hardness, measure density. Then come optical instruments such as the refractometer (which measures how light bends), the polariscope and the loupe or microscope, which reveal inclusions and treatments.

Gemmological laboratories such as GIA's add spectroscopy: UV-visible to understand what causes the colour, infrared to spot certain treatments, Raman to identify a stone or its inclusions with a laser. Chemical analysis by X-ray fluorescence or laser ablation measures trace elements. In research, X-ray diffraction reveals the crystal structure itself.

The same colour can hide very different stones. That is why a name is never enough: you need measurements.

Minerals are sorted by chemistry, more precisely by the group of atoms that structures their formula. Silicates, built around silicon and oxygen, dominate: according to Encyclopædia Britannica, they make up about 95% of Earth's crust and upper mantle. Quartz, feldspars (labradorite, moonstone, amazonite), garnets, tourmalines and beryls (aquamarine, morganite) all belong to them.

The other classes are rarer but well represented in our bracelets: carbonates (malachite, rhodochrosite), sulfates (celestite, angelite), phosphates (apatite, turquoise), oxides (hematite), halides (fluorite), sulfides (pyrite). Every new species must be approved by an IMA commission before it joins the official list.

Mineralogy is not just a collector's science. The ultramarine blue of Renaissance painters came from ground lapis lazuli, before it was made in the laboratory in the 19th century. Quartz has a special property, piezoelectricity: it vibrates at a very stable frequency when a current is applied. The first quartz wristwatch sold to the public, the Seiko Astron, went on sale on 25 December 1969.

Hematite is one of the main iron ores, fluorite gave its name to fluorescence and is used as a flux in steelmaking, apatite supplies the phosphate in fertilisers. Many of the objects around us begin in a mine.

Mineralogy describes what a stone is: its composition, structure, hardness and geological history. It can be checked by measurement. Lithotherapy belongs to tradition: it records the qualities that different cultures have associated with stones over time. These associations are not scientifically proven and never replace medical advice.

On our stone pages the two views are kept apart: the mineral profile gives verifiable facts, the tradition section says what the stone is traditionally associated with. Knowing both lets you choose a stone knowing what you wear. We go through this line, belief by belief, in our guide Mineralogy and lithotherapy: where is the line?