Atoms, Elements, Molecules & Compounds
All matter — every gemstone, every mineral — is composed of atoms. An atom is the smallest unit of a chemical element that retains the properties of that element. Each atom has a nucleus (protons + neutrons) orbited by electrons.
Select an atom to explore its structure in 3D. Drag to rotate.
Of the 90 naturally occurring elements, just eight make up the bulk of the Earth's crust — and therefore the bulk of our gemstones:
| Element | Symbol | Atomic No. | Valency | Role in Gems |
|---|---|---|---|---|
| Oxygen | O | 8 | 2 | Present in nearly all gem minerals (oxides, silicates) |
| Silicon | Si | 14 | 4 | Backbone of silicates — quartz, beryl, garnet, tourmaline |
| Aluminium | Al | 13 | 3 | Corundum (ruby/sapphire), beryl, feldspar, spinel |
| Iron | Fe | 26 | 2, 3 | Colour agent in peridot, almandine garnet, tourmaline |
| Calcium | Ca | 20 | 2 | Feldspar (plagioclase), apatite, calcite |
| Sodium | Na | 11 | 1 | Feldspar (albite), jadeite |
| Potassium | K | 19 | 1 | Orthoclase feldspar (moonstone) |
| Magnesium | Mg | 12 | 2 | Peridot, pyrope garnet, spinel |
Valency
Electron orbits surround the nucleus in concentric shells, each an energy level. The outermost shell is the valence shell. Elements form compounds by gaining, losing, or sharing electrons to achieve a stable configuration of 2 or 8 electrons in their outer shell.
Example — Corundum (Al₂O₃): Aluminium has 3 outer electrons, oxygen has 6 and needs 2 more. Two aluminium atoms (2 × 3 = 6 electrons) combine with three oxygen atoms (3 × 2 = 6 needed) → Al₂O₃ — ruby and sapphire.
Variable valency: Some elements like iron have more than one valency. Fe²⁺ (ferrous) forms different compounds than Fe³⁺ (ferric) — this is why heat treatment can change a gemstone's colour by converting Fe²⁺ ↔ Fe³⁺.
Noble gases (helium, neon, argon) already have stable outer shells (2 or 8 electrons) — they have zero valency and are virtually inert.
Bonding
The chemical joining of elements into a compound is called bonding. Two types are critical in gemology: ionic and covalent.
Van der Waals' bonds are very weak bonds. In graphite, three of carbon's four bonds are strong covalent bonds forming sheets, but the fourth is a weak van der Waals' bond — explaining why graphite is soft and slippery, while diamond (all covalent) is the hardest substance known.
Metallic bonding occurs in metals where valence electrons roam freely between positive ions. This makes metals excellent conductors of electricity and heat. Diamond is unique: an electrical insulator yet conducts heat better than most metals.
Bonding & Valency in Gemstones
See how atoms combine using their valencies to form real gemstone compounds. Each 3D model shows the atoms, electron transfer or sharing, and the resulting bond type.
Gemstone Chemistry by Group
Gemstones can be classified into four chemical groups based on their composition. Each group has characteristic properties related to hardness and durability.