Some customers keep significant amounts of stone on hand. Quantities are determined on a regular basis, then ordered and received, allowing for customers to browse and select stone from stock before confirming a Job. Material is assigned from stock once a Job is confirmed. That allocated Material is consumed, leftover remnants may be placed back into stock.
If this sounds like your workflow, add a few Suppliers and a Purchase Order Form. Create an order and receive slabs, then create a test Job to step through allocating and consuming slabs.