Following proper procedure is critical to prevent jam or jelly from becoming grainy. This video describes causes as well as tips to prevent crystal formation.
Low methoxyl pectin, if correctly used, will create a gel in the absence of sugar and acid for those desiring jams and jelly containing less sugar.
Fruits have varying levels of acidity. The higher the acid level the better the gelling process, low acid fruits require additional acid to obtain a good gel.