Of most importance to the 3D artist in the rendering of a gem model are it's color and refraction settings. While theoretically, in 3D renders, any combination is possible, such as a blood-red diamond or lime-green ruby, the following charts provide information on the the natural characteristics of gemstones.
(It is assumed that the transparency settings is 100%.)
GEMSTONE

REFRACTIVE
INDEX

COLOR
Diamond

2.417

Normal: Clear, colorless
Variants: Light shades of yellow, brown, pink
Extremely rare: Blue, green, red, violet
Ruby

1.76

Normal: Red
Nomenclature reserves the term 'ruby' for the true red colored stones, and all other colored variants are termed sapphire, [ex. 'pink' sapphire]
(See notes on Sapphire)
Sapphire 1.76 Normal: Blue
(Note: This is, except for color, the same stone as ruby.)
Variants: Pink, violet, green, colorless
Chrysoberyl 1.75

Normal: Golden yellow

'Red' Spinel 1.71 Normal: Red to red-violet
'Blue' Spinel 1.71 Normal: Blue
Variants: Light clear blue to sooty-dark blue-violet
Topaz 1.61 Normal: Golden yellow
Variants: Light pink, light blue, colorless
Emerald (Beryl) 1.57 Normal: Green
Aquamarine (Beryl) 1.57 Normal: Sky-blue to blue-green
Beryl 1.57 Normal: Greens to yellows
Garnet (Pyrope) 1.73-1.75 Normal: Bright to deep red
Garnet (Rhodolite) 1.76 Normal: Deep pink to pinkish-red
Garnet (Almandine) 1.76-1.83 Normal: Red to deep red-violet
Zircon 1.96

Normal: Colorless
Variants: Red, blue, yellow, brown, green
Tourmaline 1.62 Normal: Grey-black
Variants: Pinks, reds, browns, yellows. Also often found in two-color crystal growths, usually green shading to pink, red or violet
Quartz (Rock crystal, amethyst, citrine, rose quartz) 1.54 Normal: Colorless (Rock crystal), purple (Amethyst), yellow (Citrine), and milky-pink (Rose quartz)
Peridot 1.65 Normal: Green
Variants: Olive to yellow-green
     

For more detailed articles on these topics, Bob Keller has several clear, but detailed, explanations available at his site. While these are geared toward actual lapidary situations, they do contain information which can be applied to 3D rendering. Two of the more relevant ones are:

PROPERTIES OF COMMONLY FACETED MINERALS AND MATERIALS

REFRACTIVE INDEX AND CRITICAL ANGLE
 
 

Especially for Bryce users:
A collection of presets for most of the above gemstone materials.
BryceGemMats.zip (290K)