From WoWWiki

Bonuses for Spell damage can only be obtained through Red gems, Orange gems and Purple gems, as well as Meta gems.

Color	Name	Effect(s)
Meta	[Ember Skyfire Diamond]	+14 Spell Damage	+2% Intellect
	[Imbued Unstable Diamond]	+14 Spell Damage	5% Stun Resistance
	[Swift Starfire Diamond]	+12 Spell Damage	Minor Run Speed Increase
Red	[Don Julio's Heart]	+14 Spell Damage
	[Runed Crimson Spinel]	+12 Spell Damage
	[Runed Ornate Ruby]	+12 Spell Damage
	[Runed Living Ruby]	+9 Spell Damage
	[Stark Blood Garnet]	+8 Spell Damage
	[Runed Blood Garnet]	+7 Spell Damage
	[Runed Tourmaline]	+5 Spell Damage
Orange	[Infused Fire Opal]	+6 Spell Damage	+4 Intellect
	[Unstable Topaz]	+5 Spell Damage	+4 Intellect
	[Mysterious Fire Opal]	+6 Spell Damage	+5 Spell Penetration
	[Potent Ornate Topaz]	+6 Spell Damage	+5 Spell Crit Rating
	[Potent Pyrestone]	+6 Spell Damage	+5 Spell Critical Rating
	[Potent Fire Opal]	+6 Spell Damage	+4 Spell Critical Rating
	[Potent Noble Topaz]	+5 Spell Damage	+4 Spell Critical Rating
	[Potent Flame Spessarite]	+4 Spell Damage	+3 Spell Critical Rating
	[Veiled Pyrestone]	+6 Spell Damage	+5 Spell Hit Rating
	[Shining Fire Opal]	+6 Spell Damage	+5 Spell Hit Rating
	[Veiled Noble Topaz]	+5 Spell Damage	+4 Spell Hit Rating
	[Veiled Flame Spessarite]	+4 Spell Damage	+3 Spell Hit Rating
Purple	[Fluorescent Tanzanite]	+6 Spell Damage	+4 Spirit
	[Glowing Shadowsong Amethyst]	+6 Spell Damage	+7 Stamina
	[Glowing Tanzanite]	+6 Spell Damage	+6 Stamina
	[Infused Amethyst]	+6 Spell Damage	+6 Stamina
	[Glowing Nightseye]	+5 Spell Damage	+6 Stamina
	[Glowing Shadow Draenite]	+4 Spell Damage	+4 Stamina