Very true, many items could be put to use replicating a mortar & pestle; the one I ordered online just makes it easier in the kitchen at home.
I'm thinking a river rock that is the right size and already smooth by tumbling in the river would make a good mortar -- roll it with the palm of your hand. A good pestle in the field would be any hard surface that would allow the mortar to roll over/crush the item and then contain the powder once ground. Simple.