Hello, we are manually crafting jewels with metal, stones... due to the manual process the quantity of metal, size of stone can be different. How can we track the quantities used (and costs) at the product level?

That sounds like a tough ask. Is there any sort of standardization? Are you build to stock or to order?

There are standardizations, but from 1 ring to an other, the quantity of metal will change due to the manual work and the stone size. And we are doing both build processes.

For different quantities, you can create bom revisions with Advanced bom, but you will need to know the size before hand, unless you make on the fly bom with suitelet

Unit of Measure for your items may need to be in Weight and not units of eaches? Otherwise Catch weights may be something to read up on..

Please note that you can change component(s) on the work order after it is defaulted from a default structure. If that is something manageable to do manually changes, you can consider as well