Fire (gemstones)
The flashes of spectral colour in a gemstone, caused by dispersion splitting white light into its colours.
The optics
White light contains every colour. Inside a gemstone, violet light bends a little more than red light, so the colours separate on their way through the stone. This separation is called dispersion. When the light leaves the crown, the separated colours reach your eye as small coloured flashes.
Gems compared
Diamond has a dispersion of about 0.044, which gives it its famous fire. Moissanite has more than twice as much, so it flashes much more colour, and cubic zirconia also shows more fire than diamond. Sapphire, ruby and emerald have low dispersion and show little fire.
Cut and lighting
Fire needs a stone cut to send light back through the crown, and a small, bright light source such as a spotlight or the sun. Under diffuse light, even a fine diamond shows little fire.
Rendering fire
Renderers create fire by tracing different wavelengths, or red, green and blue separately, at slightly different IOR values. Use the dispersion value that matches the stone type. Turning dispersion far above the real value creates a rainbow effect that looks fake and misrepresents the stone, which is a problem in a product listing.
Fire is a moving effect, so a 360° video or turntable shows it far better than a still. Small, bright lights in the gem environment also help the coloured flashes appear.