Here is a link to an article on crystallisation of honey by Khalil Hamdan Apeldoorn, the Netherlands. Hamish passed it on to me, though he says the original source was Sheila. Hamish says he thinks it will be of particular interest to beginners. It contain a very nice table which will alert you to which honeys are liable to crystallise quickly [and be difficult to extract] and which are not. I had certainly heard of oil seed rape as a honey needing early extraction , but there are quite a few others in this category.