Silicate.

Silicates are the dominant family of minerals on earth. They make up roughly ninety percent of the planet's crust by volume and account for most of the gemstones a buyer is likely to meet. The defining feature is structural: every silicate is built around a basic unit of one silicon atom bonded to four oxygen atoms in a tetrahedron, written SiO4 with a charge of negative four. The tetrahedron is small (about 0.26 nanometres on a side), highly stable, and capable of bonding to itself by sharing oxygen corners with neighbouring tetrahedra. How those tetrahedra connect (in isolated units, in pairs, in rings, in chains, in sheets, or in three-dimensional frameworks) defines the sub-family and most of the physical properties that follow.

The six silicate sub-classes

Mineralogists divide silicates into six structural sub-classes based on how the SiO4 tetrahedra link together. Nesosilicates (or orthosilicates) keep their tetrahedra isolated, sharing oxygens only with non-silicate cations: olivine (the gem variety is peridot), garnet, zircon, kyanite, andalusite, and topaz all sit here. Sorosilicates pair two tetrahedra into a shared-oxygen unit, as in epidote and tanzanite (the blue gem variety of zoisite). Cyclosilicates link tetrahedra into rings of three, four, or six: tourmaline, beryl (emerald, aquamarine, morganite, heliodor), and cordierite (iolite) are the well-known examples. Inosilicates form single or double chains, including the pyroxenes (jadeite, spodumene which gives kunzite and hiddenite) and the amphiboles (nephrite jade, actinolite, hornblende). Phyllosilicates stack tetrahedra into sheets, producing the platy minerals: micas (muscovite, biotite, lepidolite), serpentine, talc, kaolinite, and chlorite. Tectosilicates build a three-dimensional framework where every oxygen is shared, the most thoroughly polymerised arrangement: quartz, the feldspars, the feldspathoids (sodalite, lazurite, leucite), and the zeolites all live here.

The structural family often predicts behaviour. Sheet silicates cleave easily along the layers, which is why mica peels and talc feels soapy. Framework silicates have no easy break direction, which is why quartz and feldspar form solid blocks. Chain silicates tend to grow as needles or fibres. Isolated tetrahedra packed with metal cations make dense, hard, often brilliantly coloured stones (peridot, garnet, zircon).

What sits outside

Outside the silicate class sit the carbonates (calcite, malachite, rhodochrosite, dolomite), the oxides (corundum which is ruby and sapphire, hematite, magnetite, spinel, chrysoberyl), the sulphides (pyrite, galena), the phosphates (apatite, turquoise, vivianite), the halides (fluorite, halite), the native elements (gold, copper, sulphur, diamond which is pure carbon), and the few stones that are not minerals at all, such as opal, amber, pearl, and obsidian (see mineraloid). Knowing the class tells the buyer something. Carbonates are softer (calcite at 3, malachite at 3.5 to 4) and react to acids including vinegar and lemon juice. Oxides include both the hardest gem material (corundum at 9, diamond at 10) and the magnetic ones (magnetite, hematite to a lesser extent). Halides like fluorite cleave perfectly and are easily damaged.

A common misconception is that "silicate" is a chemistry of pure silica. It is not. Silica (SiO2, quartz) is one specific silicate. Most silicates are mixed compounds where silicon, oxygen, and a host of other cations (aluminium, magnesium, iron, calcium, sodium, potassium) come together in different proportions. Another misconception is that the family name says something about energetic properties in modern crystal practice. It does not, in any direct way. For day-to-day crystal work the silicate label is mostly useful as orientation. Knowing that a stone belongs to the silicates often gives a quick read on its hardness (most silicate gems sit between 5.5 and 8), durability, and likely behaviour around water and sunlight. It is the family tree, not the personality.