Picture above credit to: Bee Somebody blog.
Beekeepers are frequently asked about honey crystalizing, what it means, if it affects the quality, and what can be done to prevent it, and can it be reversed. The short answers are: It simply means the sugars in the honey have come out of the liquid state and formed crystals. Honey is a product of the nectar of flowers which vary in their ratios of types of sugars. Nectars with high glucose to fructose ratios tend to crystalize quickly. Here in South Carolina cotton honey is often sought after however it crystalizes quickly, sometimes in just a couple months. Tupelo, on the other hand, may last years. Regarding quality, crystallization is not a reflection on quality one way or the other. In some countries crystalized honey is sought after and used as a spread. Crystallization is simply a process that occurs based on the ratio of sugars in the honey. To prevent or delay crystallization keep honey at room temperature or in a cupboard. Never keep honey in the refrigerator which is close to the ideal temperature to promote crystallization. Finally, to reverse crystallization, simply place your jar of crystalized honey in a pan of warm water. The warming process should be a gentle and patient warming. I tell people to do this at night just prior to going to bed and they will wake up to a jar of liquid honey ready for use at their breakfast table. Never rush the process or attempt microwaving the honey or enzymes and other healthy properties are destroyed, or worse, plastic bottles can melt and contaminate your precious honey.
More information can be from found on the web page Benefits of Honey.