This is a subject I have taken some personal interest in without studying the subject.
Here are my thoughts.

1. Put rocks in a fire and test which sort withstand the heat.
2. Choosing these most heat resistant grains… using like with like in a rotational fashion… using friction… take the same rock as a grinding head to bore down into another rock of the same heat resistant type… creating a crucible.

The next issue will be finding metal ore

Tim Wikiriwhi.