MINERALOGY · 52 GUIDES

Fifty-two minerals, one guide at a time.

Mohs hardness, crystal system, color range — from mineralogy references.

Tournament · GemstonesTake the Gemstones World Cup
Almandine garnet specimen showing characteristic deep red colour and dodecahedral crystal form

Almandine Garnet

Most common garnet species; deep red to reddish-purple iron aluminium silicate; Mohs 7–7.5.

Hardness
7.25 / 10
Color
deep red
Amethyst crystal cluster from Magaliesburg, South Africa showing purple colour

Amethyst

Purple quartz variety coloured by iron impurities and irradiation; Mohs 7.

Hardness
7 / 10
Color
purple
Brilliant-cut diamond showing characteristic sparkle and fire representing April's birthstone

April Birthstone

Diamond — hardest natural substance; brilliance and eternity for the spring month.

Hardness
—
Color
colourless (D-F grade)
Aquamarine crystal cluster on albite matrix showing blue-green colour

Aquamarine

Blue-green beryl coloured by ferrous iron impurities, Mohs 7.5–8.

Hardness
7.75 / 10
Color
blue
Peridot (gem olivine) crystals showing characteristic lime green colour representing August's birthstone

August Birthstone

Peridot — volcanic olivine; lime green gem of the sun and the ancient island of Zabargad.

Hardness
—
Color
lime green
Precious opal from Australia showing play-of-colour on dark body

Black Opal

Precious opal with dark body colour (N1-N4) maximising play-of-colour contrast; Lightning Ridge primary.

Hardness
6 / 10
Color
black
Brilliant-cut diamond viewed face-up showing the 57-facet arrangement

Brilliant Cut

57-58 facet round cut maximising light return; proportions standardised by Marcel Tolkowsky (1919).

Hardness
—
Color
—
Polished cabochon-cut precious opal showing smooth dome form typical of cabochon cutting

Cabochon

Smooth domed gem cut without facets; oldest cutting style; suited for opaque and phenomena gems.

Hardness
—
Color
—
Chrysoberyl crystal; the cat's eye variety is cut as a cabochon to display chatoyancy

Cat's Eye Chrysoberyl

Chatoyant chrysoberyl with a single reflective band; trade name cymophane; Sri Lanka primary source.

Hardness
8.5 / 10
Color
honey-yellow
Chrysoberyl crystal cluster showing orthorhombic crystal habit and yellow-green colour

Chrysoberyl

Beryllium aluminate; Mohs 8.5, orthorhombic, includes alexandrite and cat's eye varieties.

Hardness
8.5 / 10
Color
yellow
Faceted citrine gemstone showing yellow-orange colour

Citrine

Yellow to orange quartz variety coloured by ferric iron; Mohs 7, trigonal system.

Hardness
7 / 10
Color
yellow
Tanzanite crystal showing characteristic blue-violet colour representing one of December's three birthstones

December Birthstone

Turquoise, tanzanite, and zircon — December's trio offers deep blue through sky blue for the winter month.

Hardness
—
Color
sky blue to blue-green (turquoise)