Crystallisation occurs when glucose separates from water in the honey, the glucose remains in the form of crystals. The levels of fructose and glucose in the composition of the honey will determine how quickly a honey will crystallise. Glucose crystallises much quicker due to its lower solubility, whilst fructose will remain in a liquid state.
A honey with higher fructose levels will remain liquid for longer than a honey with high glucose levels.
To reverse the process place the jar of honey with the lid tightly on in a bowl of warm water and the glucose crystals will return to liquid. This does not affect the quality or delicious flavour of the honey.