Coal
Your furnaces and forges in Graveyard Keeper need fuel to run. It can be ordinary sticks (in large quantities), slightly more efficient firewood and the best source of fuel - coal, which is much more in the game world, but it takes a little more time to get this resource.
First, you need to repair the damaged bridge to the left of the graveyard that leads to the swamp. The repair requires 3 wood planks, 8 wood wedges and 12 nails.
Once you have repaired the bridge, cross it and take the dirt road north and then turn left. Eventually, you will come across rubble on the road that needs to be removed. You will need 10 wooden wedges, 4 simple iron pieces and 2 wooden planks (in the picture below, the path is already cleared).
Continue north through the trees until you reach Quarry. Now go a little east, where you will find a large coal deposit.
Yes, it takes longer to get here, but each mining will give you 20 to 30 coal. After spending a few minutes here, you can get hundreds of coal in one go.
Iron ore
At the beginning of the game, iron ore is fairly easy to find: it is in the area north of your house, and even more ore will be in the swamp to the left of your graveyard.
But eventually these iron deposits will be depleted and you will need to find a more permanent source of the resource. It's next to coal, so just follow the directions above.
Note that this deposit never ends.
Marble
To create more beautiful gravestones, grave barriers and statues to increase the rating of the cemetery, you will need to get some marble. This resource is right next to the iron and coal deposits, so follow the instructions for mining coal.
Interact with the marble source, then select Build a Marble Quarry with 16 Wooden Wedges, 4 Simple Iron Parts and 7 Nails. Once done, you can use the 5 wood wedges created in the wood grinder to make two marble slabs each time.
Unlike iron and wood, marble slabs do not need a separate storage space - a storage for the stone that you have from the very beginning of the game is fine. You can then use a stone grinder to turn the slab into smaller pieces.