The trick is to shatter them with the proper enchanting recipe. One crystal will turn into up to 12 Infinite Dusts or 6 Greater Cosmic Essences. For a while they will still give you some nice profit. Currently a Cosmic essence goes for 8g each and a stack of Infinite Dusts 45g, if you first buy the cheaper 19g stacks from the way :) As you can see, it's a very easy way to make some profit and only takes a few minutes from your time.
Additionally you can turn the crystals, dusts and essences into something even more useful. Here's a list of enchants I've found worthwhile. Most of them are needed by level 70 and 60 characters that have just obtained their shiny brutal gladiator or older grand marshal/warlord PVP gear. The wrath recipes are obtained from the enchant seller in Dalaran. Note that one character usually needs only one of these enchants in their time, so do not expect them to sell instantly. Fiery Weapon enchant is an exception though.
Tip: It's wise to create only one scroll of each enchant at a time.
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Enchant weapon: Black Magic
Requires: Greater Cosmic Essence x 6, Dream Shard x 6, Abyss Crystal x 6
Sell for: 350g
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Enchant weapon: Berserking
Requires: Infinite Dust x 12, Greater Cosmic Essence x 4, Dream Shard x 4, Abyss Crystal x 10
Sell for: 399g
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Enchant weapon: Massacre
Requires: Infinite Dust x 40, Greater Cosmic Essence x 6, Abyss Crystal x 6
Sell for: 350g
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Enchant weapon: Mighty Spellpower
Requires: Infinite Dust x 30, Dream Shard x 6, Abyss Crystal x 6
Sell for: 350g
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And a bonus!
-------------------------------------------------------------------------------------Enchant weapon: Fiery Weapon
Obtainable from: Pyromancer Loregrain (Or AH)
Requires: Small Radiant Shard x 4, Essence of Fire x 1
Sell for: 65g
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