Mineral, variety, rock, glass
A mineral is a natural substance with a defined chemical composition and an ordered internal structure. Quartz has the composition SiO₂, and amethyst, citrine, rose quartz, smoky quartz and rock crystal are its varieties — they differ in colour, not species. Lapis lazuli is a rock made of several minerals, so it has no single chemical formula. Obsidian is a natural volcanic glass with no crystal structure. Garnet is a group of minerals, and labradorite a name for a compositional range of plagioclase. In the atlas the material type is shown beside the name. [1] [31] [20] [34] [79] [22] [32]
See in Avonoria: Rock crystal · Lapis lazuli · Obsidian · Garnet
Crystal systems
The ordered structure of minerals belongs to one of the crystal systems. The atlas includes representatives of many of them: cubic (fluorite, pyrite, sodalite — hence the cubes of pyrite and fluorite), trigonal (quartz, tourmaline, hematite), hexagonal (beryl, and so aquamarine), monoclinic (gypsum, i.e. selenite, malachite, jadeite) and triclinic (labradorite, amazonite). For quartz we give “trigonal”, following the Australian Museum and the symmetry in the Handbook of Mineralogy; older tables use the broader hexagonal family. [5] [9] [4] [1] [31] [2] [12] [8] [10] [6] [7] [3] [11]
See in Avonoria: Fluorite · Pyrite · Aquamarine
Mohs hardness
The Mohs scale describes resistance to scratching, from 1 (talc) to 10 (diamond); the steps are not equal. In the atlas the softest is selenite (1.5–2), then malachite (3.5–4), fluorite (4), obsidian (5–5.5), lapis lazuli and hematite (5–6), sodalite (5.5–6), the feldspars — labradorite, moonstone, amazonite — and pyrite (6–6.5), jadeite (6–7), garnets (6.5–7.5), quartz and tourmaline (7) and aquamarine (7.5–8). A harder stone will scratch a softer one, which is why they are stored separately. Hardness is not the same as toughness: moonstone is harder than fluorite but can break from a knock because of its cleavage. [10] [6] [5] [79] [12] [4] [3] [9] [7] [22] [1] [2] [8] [27]
See in Avonoria: Cleaning and care
Cleavage, fracture, streak and lustre
Cleavage is a tendency to break along flat planes: fluorite has perfect octahedral cleavage, gypsum perfect cleavage in one direction, feldspars in two. Fracture describes surfaces other than cleavage; quartz and obsidian break conchoidally, giving sharp edges. Streak is the colour of a mineral’s powder: hematite gives a red-brown streak despite its steel-grey surface, and pyrite a greenish-black one. Lustre describes how light is reflected — vitreous in quartz and obsidian, metallic in pyrite and hematite, pearly in gypsum. [5] [10] [155] [1] [79] [12] [9]
Optical effects and colour
Some atlas stones are famous for optical effects. Labradorite and moonstone are feldspars known for their play of light; GIA links moonstone’s glow to light scattering in the layered feldspar structure. Colour often comes from impurities or defects: smoky quartz owes its colour to natural irradiation, and amethyst and citrine can change colour with heat — hence heated amethyst sold as citrine. Colour helps when browsing a collection, but on its own it is not proof of identity. [19] [71] [1] [18]
See in Avonoria: Labradorite · Moonstone · Smoky quartz
Where stones form
Quartz forms in many settings — hydrothermal veins, pegmatites and igneous rocks — which is why it is so common. Obsidian is lava that cooled so quickly that crystals had no time to form. Malachite forms in the oxidised zones of copper deposits, labradorite is a component of igneous rocks such as anorthosites, and pyrite forms in hydrothermal, sedimentary and metamorphic settings. Where and how a stone formed explains many features of a specimen: inclusions, banding, crystal shape. [1] [31] [34] [79] [6] [3] [9]
Read next
Natural stone, treatment or imitation?Cleaning and careMineral safetyA history of stones: objects and beliefsCrystal atlas →Guide sources
- Quartz (external link, opens in a new tab)Mineral Data Publishing / Handbook of Mineralogy · accessed 2026-09-30
Mineralogical, gemological, institutional or scientific source. Its scope is recorded in the source index.
- Quartz gemstones (external link, opens in a new tab)Australian Museum · 2026 · accessed 2026-09-30
Mineralogical, gemological, institutional or scientific source. Its scope is recorded in the source index.
- Lapis lazuli (external link, opens in a new tab)Gemological Institute of America · accessed 2026-09-30
Mineralogical, gemological, institutional or scientific source. Its scope is recorded in the source index.
- Obsidian Cliff (external link, opens in a new tab)US National Park Service · accessed 2026-09-30
Mineralogical, gemological, institutional or scientific source. Its scope is recorded in the source index.
- Obsidian — Volcano World (external link, opens in a new tab)Oregon State University · accessed 2026-09-30
Mineralogical, gemological, institutional or scientific source. Its scope is recorded in the source index.
- Garnet (external link, opens in a new tab)Gemological Institute of America · accessed 2026-09-30
Mineralogical, gemological, institutional or scientific source. Its scope is recorded in the source index.
- Labradorite: mineralogy of Wales (external link, opens in a new tab)Amgueddfa Cymru / Museum Wales · accessed 2026-09-30
Mineralogical, gemological, institutional or scientific source. Its scope is recorded in the source index.
- Fluorite (external link, opens in a new tab)Mineral Data Publishing / Handbook of Mineralogy · accessed 2026-09-30
Mineralogical, gemological, institutional or scientific source. Its scope is recorded in the source index.
- Pyrite (external link, opens in a new tab)Mineral Data Publishing / Handbook of Mineralogy · accessed 2026-09-30
Mineralogical, gemological, institutional or scientific source. Its scope is recorded in the source index.
- Sodalite (external link, opens in a new tab)Mineral Data Publishing / Handbook of Mineralogy · accessed 2026-09-30
Mineralogical, gemological, institutional or scientific source. Its scope is recorded in the source index.
- Schorl (external link, opens in a new tab)Mineral Data Publishing / Handbook of Mineralogy · accessed 2026-09-30
Mineralogical, gemological, institutional or scientific source. Its scope is recorded in the source index.
- Hematite (external link, opens in a new tab)Mineral Data Publishing / Handbook of Mineralogy · accessed 2026-09-30
Mineralogical, gemological, institutional or scientific source. Its scope is recorded in the source index.
- Beryl (external link, opens in a new tab)Mineral Data Publishing / Handbook of Mineralogy · accessed 2026-09-30
Mineralogical, gemological, institutional or scientific source. Its scope is recorded in the source index.
- Gypsum (external link, opens in a new tab)Mineral Data Publishing / Handbook of Mineralogy · accessed 2026-09-30
Mineralogical, gemological, institutional or scientific source. Its scope is recorded in the source index.
- Malachite (external link, opens in a new tab)Mineral Data Publishing / Handbook of Mineralogy · accessed 2026-09-30
Mineralogical, gemological, institutional or scientific source. Its scope is recorded in the source index.
- Jadeite (external link, opens in a new tab)Mineral Data Publishing / Handbook of Mineralogy · accessed 2026-09-30
Mineralogical, gemological, institutional or scientific source. Its scope is recorded in the source index.
- Labradorite (external link, opens in a new tab)Mineral Data Publishing / Handbook of Mineralogy · accessed 2026-09-30
Mineralogical, gemological, institutional or scientific source. Its scope is recorded in the source index.
- Microcline (external link, opens in a new tab)Mineral Data Publishing / Handbook of Mineralogy · accessed 2026-09-30
Mineralogical, gemological, institutional or scientific source. Its scope is recorded in the source index.
- Moonstone care and cleaning (external link, opens in a new tab)Gemological Institute of America · accessed 2026-09-30
Mineralogical, gemological, institutional or scientific source. Its scope is recorded in the source index.
- Sunstone care and cleaning (feldspar) (external link, opens in a new tab)Gemological Institute of America · accessed 2026-10-04
Mineralogical, gemological, institutional or scientific source. Its scope is recorded in the source index.
- Moonstone (external link, opens in a new tab)Gemological Institute of America · accessed 2026-09-30
Mineralogical, gemological, institutional or scientific source. Its scope is recorded in the source index.
- Moonstone Description (external link, opens in a new tab)Gemological Institute of America · accessed 2026-09-30
Mineralogical, gemological, institutional or scientific source. Its scope is recorded in the source index.
- Citrine description (external link, opens in a new tab)Gemological Institute of America · accessed 2026-09-30
Mineralogical, gemological, institutional or scientific source. Its scope is recorded in the source index.