“Virtually all types of honey will crystallize in time. Some types of honey may crystallize very rapidly, while others may remain liquid for years.
Sometimes, though, naturally crystallized honey granulates with coarse crystals, resulting in a product that most people wouldn’t care to eat. But the natural crystallization process can be controlled to create a delectable product usually called creamed honey, or spun honey.
One unfortunate byproduct of the natural crystallization process can be fermentation. As mentioned above, the crystallized honey is part liquid and part solid. Since the solid crystals contain no water, the remaining liquid portion of the honey contains a higher concentration of water than before crystallization.
This makes the honey more susceptible to fermentation, and when this happens, the honey is ruined.
But if you have some crystallized or granulated honey that you wish to liquefy – and assuming it hasn’t fermented – it’s very easy to restore it to a liquid state. Simply place the jar of honey in a pan of warm water, and leave it until the heat has melted all of the honey crystals.
Just take care not to get the honey warmer than necessary, because that will damage the flavor of the honey.
If the water temperature is just on the verge of being too hot for you to put your hand in it, then that’s about the right temperature for the honey. If it’s too hot for your hand, it’s too hot for the honey.”
Source: http://www.bees-and-beekeeping.com/crystallized-honey.html