Parry gems are red gems, orange gems, purple gems, and prismatic gems.

Northrend Gems

Color	Name	Effect(s)
Prismatic	[Flashing Dragon's Eye]	27 Parry Rating
Red	[Flashing Dragon's Eye] †	+34 Parry †
	[Flashing Cardinal Ruby]	+20 Parry Rating
	[Flashing Scarlet Ruby]	+16 Parry Rating
	[Perfect Flashing Bloodstone]	+14 Parry Rating
	[Flashing Bloodstone]	+12 Parry Rating
Orange	[Glimmering Ametrine]	+10 Parry Rating	+10 Defense Rating
	[Glimmering Monarch Topaz]	+8 Parry Rating	+8 Defense Rating
	[Perfect Glimmering Huge Citrine]	+7 Parry Rating	+7 Defense Rating
	[Glimmering Huge Citrine]	+6 Parry Rating	+6 Defense Rating
Purple	[Defender's Dreadstone]	+10 Parry Rating	+15 Stamina
	[Defender's Twilight Opal]	+8 Parry Rating	+12 Stamina
	[Defender's Shadow Crystal]	+6 Parry Rating	+9 Stamina

^† In Patch 3.2.0.

Outland gems

Color	Name	Effect(s)
Red	[Flashing Crimson Spinel]	+10 Parry Rating
	[Flashing Living Ruby]	+8 Parry Rating
Orange	[Glimmering Fire Opal]	+5 Parry Rating	+4 Defense Rating
	[Splendid Fire Opal]	+5 Parry Rating	+4 Resilience Rating
Purple	[Defender's Tanzanite]	+5 Parry Rating	+6 Stamina

Notes

Before Patch 3.2.0 in particular, parry gems are generally considered inferior to dodge gems or defense gems because of the amount of avoidance they give when taking into account diminishing returns. This still may be true after the patch is released, but it should be less so.

Recently, Greg "Ghostcrawler" Street, the Lead Systems Designer posted about this very topic on the forums:

Re: Avoidance Change | 2009-07-13 21:28 | Ghostcrawler

Q u o t e:

However, as far as the dodge vs parry, you've got to consider that a lot of people think in absolutes. "Parry is not as good as dodge" turns into "NEVER GEM PARRY BECAUSE IT'S A GARBAGE STAT". I don't think it'll change their perceptions, but that's more of a problem with humanity.

Oh yes. At some point "Not BiS" became "trash." :)

It's not the end of the world if parry gems rot. But we want to be able to put parry on say rings without those just being sharded.

View original post

Patch changes

• Patch 3.2.0 (04-Aug-2009):
  • [Dragon's Eye] gems stat bonuses increased.
  • We will be removing the prismatic quality of the jewelcrafter-only [Dragon's Eye] gems. 
    

v • d • e Socket gems By material Red [Blood Garnet] · [Living Ruby] · [Crimson Spinel] [Bloodstone] · [Scarlet Ruby] · [Cardinal Ruby] [Carnelian] · [Inferno Ruby] · [Queen's Garnet]† [Tiger Opal] · [Primordial Ruby] Orange [Flame Spessarite] · [Noble Topaz] · [Pyrestone] [Huge Citrine] · [Monarch Topaz] · [Ametrine] [Hessonite] · [Ember Topaz] · [Lava Coral]† [Pandarian Garnet] · [Vermilion Onyx] Yellow [Golden Draenite] · [Dawnstone] · [Lionseye] [Sun Crystal] · [Autumn's Glow] · [King's Amber] [Alicite] · [Amberjewel] · [Lightstone]† [Sunstone] · [Sun's Radiance] Green [Deep Peridot] · [Talasite] · [Seaspray Emerald] [Dark Jade] · [Forest Emerald] · [Eye of Zul] [Jasper] · [Dream Emerald] · [Elven Peridot]† [Alexandrite] · [Wild Jade] Blue [Azure Moonstone] · [Star of Elune] · [Empyrean Sapphire] [Chalcedony] · [Sky Sapphire] · [Majestic Zircon] [Zephyrite] · [Ocean Sapphire] · [Deepholm Iolite]† [Lapis Lazuli] · [River's Heart] Purple [Jaggal Pearl] · [Shadow Pearl] · [Shadow Draenite] · [Nightseye] · [Shadowsong Amethyst] [Shadow Crystal] · [Twilight Opal] · [Dreadstone] [Nightstone] · [Demonseye] · [Shadow Spinel]† [Roguestone] · [Imperial Amethyst] Prismatic [Northsea Pearl] · [Dragon's Eye] · [Siren's Tear] [Chimera's Eye] [Serpent's Eye] Meta [Earthstorm Diamond] · [Skyfire Diamond] · [Starfire Diamond] · [Unstable Diamond] [Earthsiege Diamond] · [Earthshatter Diamond] · [Skyflare Diamond] · [Starflare Diamond] [Shadowspirit Diamond] [Primal Diamond] Cogwheel [Flashing] · [Fractured] · [Precise] · [Quick] · [Rigid] · [Smooth] · [Sparkling] · [Subtle] By category By stats Agility · Intellect · Spirit · Stamina · Strength · Attack Power · Critical strike · Hit rating · Haste rating · Resilience · Spell Power · Spell Penetration · Mana every 5 Seconds · Threat reduction · Defense · Dodge · Parry By source Vendors · Factions · PvP · Quests · Enchanting By quality/stats Uncommon · Perfect uncommon · Rare · Epic Uncommon · Perfect uncommon · Rare · Epic † Added in Patch 4.3.