Hi,
I think you would be better investing in an electric coffee grinder instead of your pestle and mortar. They are about ?15 and grind anything to a fine powder. Dry roasting the spices first is a must anyway but I don't think you will get a fine powder from the whole spices such as Cardamom, Cloves, Nutmeg etc.
I find that although home made G. Mas is so much more fragrant than shop bought it can also ruin a dish if you use too much..............err on the side of caution.
East End, Supreme and Natco do a good shop bought alternative.
Regards
Ray