Inventory

Hello, in need of some help. I work with a countertop fabrication shop, and when running a P & L report my COGS is currently negative. The main issue I believe is my bills, like mentioned we work with countertops, so we consistently purchase full size slabs of Quartz, Granite etc. and use it for jobs. We don't necessarily use the whole slab for every job therefore I can't put the actual product (i.e. slab) on the customers invoice because it will count for the whole slab and that is not the case for most jobs. I currently categorize vendors bills for slabs under COGS-Supplies and Materials. I also make each new slab/color/material into an inventory item which I believe is where I'm making a mistake. I just need to know how I should be categorizing the slabs, should I be doing 'Material" instead of COGS? Can anyone please give some insight? I know I'm all over the place so please let me know if you need further information.