I often get asked why it 'candies'; it is a quite a misunderstood phenomenon. Some people question whether this process means that it is an inferior product, poor quality or storage, or that it has been processed in some way. It is not. It is not off, it hasn't gone bad, so please do not chuck it out!! Crystallization is completely natural, and remember that honey never goes off. The oldest honey that was still edible found was 60,000 years old - we will all be heads in jars before the honey goes off!
All raw honey will candy over time. Some on-comb inside the hive (such as a clover or canola) and others take many years, if at all (such as Jarrah). Honey is dehydrated nectar (which is ~80% water). The bees dehydrate it to between ~12-16% water, the rest of the honey being combination of naturally occuring sugars (fructose and glucose). The granulation of honey occurs when the saturation of glucose within the remaining water of the honey has occurred and there is an overabundance of glucose molecules in the honey that form crystals.
Because of the slightly different chemical compositions of different nectars, each honey is slightly different in the resulting balance of natural sugars. Those with a slightly higher glucose amount will candy faster. Raw honeys generally have a low GI index of 28-32, whereas a homogenized/pasteurized (heated) honey will be about 65-75. Heating honey will delay the granulation of honey, not erase it completely. Heating does degrade the micro-nutrients and pollens that are in the honey, and the other qualities that make it a healthy sugar alternative.
The quality of crystallized honey does not change, only the texture. Some people actually prefer the grainy, thicker texture of a candied honey. Interestingly, honey can also slowly change colour (generally darker) and taste over time as well, similar to how wine can change over time.
Candied honey can be much easier for things like cups of tea or kids (non-drippy) and cooking (easier to measure for small quantities), however, if you prefer the liquid gold pop it in a pot of hot water or out in the sunshine on a warm day (with the lid well sealed). The ambient temperature over the day, at about 25 degrees or more, should bring it back to liquid by the time you get home from work! Do not put it in the microwave - it will be too hot, and if you don't gentle warm it, the crystalizing process will be faster the second time around.