The most useful predictor of the garnulation propensity of honey is the ratio of Glucose to Water. If this is above about 2 in honey with 20% moisture, granulation will likely be rapid. Nectars of Brassica (cole) crops are rich in Glucose and result in honeys that granulate quickly. The rule of thumb has some caveats as the water activity comes into play at very rich sugar concentrations, and other components of honey may influence granulation. For more details you should consult Eva Crane's encyclopaedic study HONEY (1975), Crane, Russak & Co., N.Y. and a chapter by White therein. Cheers, Peter