The day Desmond had gone through the elemental epiphany had greatly increased his understanding and affinity for four elements: Wind, Fire, Water, and Metal.
Actually, given his innate talents coupled with Kyuru's blessing, Desmond could theoretically use any element of mana with proper practice but it was common sense to specialize in those elements with whom he had the greatest affinity.
And of the four elements, Desmond had a greater affinity for metal followed by wind and then water, leaving fire as the element with which he had the least affinity.
The elemental affinities were something that depended on the talent and personality of a person and although Desmond didn´t know the elemental affinities could also, in turn, affect the personality of a person.