65 stones

Gem library

The visual encyclopedia of gemstones. For each one: a full mineralogical fact sheet, its geological formation, its deposits, how to recognise it, which imitations to avoid, how to care for it — and what tradition attributes to it.

Agate Quartz with a thousand ribbons Mohs 6.5 – 7 Alexandrite Emerald by day, ruby by night Mohs 8.5 Amazonite The water-green feldspar Mohs 6 – 6.5 Amber Fossil resin, a capsule of time Mohs 2 – 2.5 Amethyst The bishops' violet quartz Mohs 7 Apatite The deceiver with a neon blue Mohs 5 Aquamarine The sailors' blue beryl Mohs 7.5 – 8 Aventurine Green quartz flecked with mica Mohs 6.5 – 7 Azurite The deep blue of oxidised copper Mohs 3.5 – 4 Black tourmaline Schorl, the protective stone par excellence Mohs 7 – 7.5 Calcite The carbonate of a thousand forms, king of birefringence Mohs 3 Carnelian The chalcedony of pharaonic seals Mohs 6.5 – 7 Charoite The swirled violet of a single river Mohs 5 – 6 Chrysocolla Copper blue-green, in velvety crusts Mohs 2.5 – 3.5 (up to 7 if silicified) Chrysoprase The nickel chalcedony Mohs 6.5 – 7 Citrine The merchants' solar quartz Mohs 7 Danburite Connecticut clarity Mohs 7 – 7.5 Diamond Pure carbon, the hardest natural material Mohs 10 Dumortierite A deep blue born in the Rhône Mohs 7 Emerald The green beryl with a garden of inclusions Mohs 7.5 – 8 Fluorite The stone that gave fluorescence its name Mohs 4 Garnet A whole family, from blood red to emerald green Mohs 6.5 – 7.5 Heliodor Gift of the sun Mohs 7.5 – 8 Hematite Black iron with a blood-red streak Mohs 5.5 – 6.5 Howlite White veined with black, queen of imitations Mohs 3.5 Iolite The Vikings' compass Mohs 7 – 7.5 Jadeite Burma's imperial jade Mohs 6.5 – 7 Jasper Opaque chalcedony of a thousand landscapes Mohs 6.5 – 7 Kunzite The evening spodumene Mohs 6.5 – 7 Labradorite The feldspar of blue lightning Mohs 6 – 6.5 Lapis lazuli Afghan midnight blue, sown with gold Mohs 5 – 6 Larimar The Caribbean set in stone Mohs 4.5 – 5 Malachite Copper green, in concentric ribbons Mohs 3.5 – 4 Moonstone Orthoclase with a floating blue veil Mohs 6 – 6.5 Morganite The banker's pink beryl Mohs 7.5 – 8 Nephrite jade The toughest stone, China's green gold Mohs 6 – 6.5 Obsidian Volcanic glass, sharper than steel Mohs 5 – 5.5 Onyx The black chalcedony of cameos Mohs 6.5 – 7 Opal Hydrated silica that holds the rainbow Mohs 5.5 – 6.5 Paraíba tourmaline Brazil's cupriferous neon Mohs 7 – 7.5 Peridot Green olivine, born in the Earth's mantle Mohs 6.5 – 7 Prehnite The grape green of lava flows Mohs 6 – 6.5 Pyrite Fool's gold, a cube of golden steel Mohs 6 – 6.5 Rhodochrosite The manganese rose, ribboned with white Mohs 3.5 – 4 Rhodonite The pink silicate veined with black Mohs 5.5 – 6.5 Rock crystal Clear quartz, the archetype of crystal Mohs 7 Rose quartz The stone of tenderness Mohs 7 Ruby Red corundum, pigeon's blood Mohs 9 Sapphire Corundum in every colour but red Mohs 9 Selenite Translucent gypsum, soft as a fingernail Mohs 2 Seraphinite Angel wings from Lake Baikal Mohs 2 – 2.5 Serpentine The snakeskin of the oceanic mantle Mohs 3 – 5 (bowenite: up to 6) Smoky quartz Quartz the colour of smoke and whisky Mohs 7 Sodalite Blue marbled with white, lapis's cousin Mohs 5.5 – 6 Sphene More fire than diamond Mohs 5 – 5.5 Spinel The gem long mistaken for ruby Mohs 8 Sugilite Magenta from the Kalahari Mohs 5.5 – 6.5 Tanzanite The blue-violet of a single hill Mohs 6 – 7 Tiger's eye The quartz that follows the light Mohs 6.5 – 7 Topaz Aluminium silicate with a formidable cleavage Mohs 8 Tourmaline The gem of mixed colours, from pink to Paraíba Mohs 7 – 7.5 Turquoise Desert blue, veined with matrix Mohs 5 – 6 Unakite The pink and green granite of the Appalachians Mohs 6 – 7 Variscite Turquoise's green cousin Mohs 4 – 5 Zircon The oldest mineral on Earth Mohs 6 – 7.5

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The gem library: a mineral encyclopedia built for the field

The gem library gathers a detailed profile for each of the 65 stones most often met by collectors, curious walkers and jewellery lovers. Every profile follows the same structure — the one a gemmologist applies in the field: first the mineralogical identity (family, species, variety, chemical formula, crystal system), then the measurable physical and optical properties: Mohs hardness, density, refractive index, birefringence, cleavage, fracture, lustre and diaphaneity.

Then comes what no table tells you: how the stone formed, in which rocks, at what depth, under which conditions — because understanding a mineral's geology is understanding why it looks the way it does. Next, the deposits and their signatures: a Uruguayan amethyst does not look like a Zambian one, and a Mogok ruby is not a Montepuez ruby.

Each profile finally gives the three most useful pieces of everyday information: the identification tests you can run at home (scratch, density, birefringence, acid reaction, UV fluorescence), the imitations and treatments to know about — dyed howlite sold as turquoise, dyed agate as carnelian, lead glass filling a ruby's fractures, resin in reconstituted malachite — and the exact care requirements: which stones fear water, salt, sunlight, ultrasonic cleaners or heat.

Traditions are included too, presented for what they are: cultural heritage, sometimes millennia old, never medicine. Combining the gem library with our other lapidary tools — the interactive Mohs scale, the identify-by-criteria tool, the colour chart, the water guide and the world gem map — takes you from looking at a pebble to a reasoned identification. And if doubt remains, Lapidem's AI identification takes over from a single photo.