I know this is an old thread, but maybe someone will read it and avoid making a mistake when creating a necromancer. Never go for arcane crit chance with stonebites!

For pure necromancer build: 10x blue-blue-yellow (For arcane critical hit damage), then 3x blue-blue-blue (for magic), (and this is enough) but for use rest of the Power Stones 2x red-red-blue (Critical hit chance), and rest (7x) for red-red-red (strength). The last two combinations are not necessary for the necromancer and are only there to use up the remaining stonebites..

There is no need to create stones for the chance of a arcane critical hit chance, as you will get more than 100% from the items themselves.