Building a hearth pad out of rocks collected locally. I've got a general idea of how to install the rocks but wouldn't mind some tips. The rocks are not brittle and pass the drop from 1 foot up test. This is the test explained to me by a mason on how to test the construction of the material your using to see if it will break under pressure. The hearh pad is going to be laid in on top of a wood subfloor. Any suggestiong on what sort of grout to use? Sandless etc.. And what sort of support pad.. cement etc should be placed underneath. Thanks.

I don't know how stabile your sub floor is but I'd start with a half inch Hardibacker board for underlayment. Glue it and screw it per manufacturer's instructions, normally modified thinset. When installing stone I don't go with grout but rather masonary cement. It does somewhat depends on what look you want. You most important step is to get the floor stabile.