Corundum

Ruby

Ruby is red corundum. Every other colour of the same mineral is sold as sapphire. Chromium gives it its red and, in daylight, a slight glow.

Ruby
Natural ruby

At a glance

Mineral: Corundum (aluminium oxide), red from chromium
Refractive index: 1.77
Dispersion (C–F): 0.011
Specific gravity: 4.00
Mohs hardness: 9
Wear: After diamond the hardest common gem: a good choice for engagement and everyday rings.

Ruby in Jart

The 2 looks in the studio's material library, with the values the renderer uses.

Optics of Ruby in Jart
Stone	Colour	RI	Dispersion C–F	Abbe	SG	Colour depth
Ruby	#d91c44	1.77	0.011	69	4.00	4.3 mm
Natural ruby	#d91c44	1.77	0.011	69	4.00	5 mm

RI refractive index (n_d). Dispersion C–F n_F − n_C, the spread between blue and red light; many gem tables quote the B–G figure instead (diamond 0.044, here 0.025). Abbe number: lower means more fire. SG specific gravity, used for carat estimates in Design Facts. Colour depth the path through the stone after which white light takes the stone's full colour; shorter paths look paler.

How it renders

With a refractive index of 1.77, a polished ruby facet reflects 7.7% of light that meets it face-on (diamond: 17.2%). Light inside the stone that meets a facet more than 34.4° from straight on cannot leave and is reflected back in; the smaller that angle, the more light a good cut returns to the eye (diamond 24.4°, quartz 40.4°).

Its dispersion (C–F) is 0.011, 45% of diamond's, which sets how much fire, the flashes of colour, a render shows under small, bright lights.

The natural ruby preset adds clouds and fine specks inside the stone, the silk a jeweller expects in an untreated stone, so a render can show a clean stone or a believable natural one.