I don't know about you, but I prefer nectarines over peaches. In my experience, the odds of getting a mealy peach is much higher than getting a mealy nectarine. In addition, nectarines are awesome because you don't have to deal with peach skin, which tastes like dryer lint.
This was one of the few jam recipes I made this summer where I actually paid for fruit, but it only cost about $3 for 3 pounds. I bought approximately 9 rock-hard nectarines and threw them into a paper bag with an old banana to speed up the ripening process. Both bananas and nectarines are climacteric fruits, which means they release ethylene while ripening (and the ethylene from the over-ripe banana helps speed up ripening of the neighboring nectarines).
I used the Ball recipe for peach/pear jam (4 cups or 3 pounds fruit, 5 cups sugar, 2 tablespoons lemon juice, and regular powdered pectin). I didn't peel the nectarines because I'm lazy and I couldn't find any skin in the finished product anyway.
I was pretty lucky with the store-bought fruit because not a single fruit was mealy. The jam tastes somewhat similar to apricot jam. In the future I want to try Pomona's Universal Pectin because I felt like the sharp nectarine flavor was a little diluted with all the sugar. In fact, next year I want to switch entirely over to Pomona's. Although my jam yield will be lower, I think I purchased almost 25 pounds of sugar this year just for jam recipes!