Honey changing into unusable, hardened clump
Honey is entirely useable after crystallization. This is a natural and spontaneous process and doesn't affect the honey negatively in terms of flavor or quality. It's dependent upon the sugar content of the particular honey, and in particular the crystallization speed is determined by:


the nectar source collected by bees (the sugar composition of honey),
the methods in which honey is handled (processed) and
the temperature in preservation.


To re-liquefy, gently heat it in a water bath in its container, up to 40&deg;C (104 &deg;F) - the temperature of a beehive in the summer. Beyond this will damage the honey.

Don't store honey in a cold area - the optimum temperature is 20-27 &deg;C (70-80 &deg;F). Ideal crystal formation occurs at 11-18 &deg;C (52-64 &deg;F), and storing in the refrigerator accelerates the process. 

More information in the PDF at the bottom of this post.

Reference:


www.montcobeekeepers.org/Documents/Honey_Crystallization.pdf