The Stone Slab is a vanilla item made by placing 3 stone next to each other on a crafting table. They are used to make Stone Ties and Player Detectors.
If you cut a stone slab with a Handsaw you can get two polished stone slabs. These will function and look the same as regular stone slabs when placed in the world, meaning that you can double the amount of stone slabs you possess for free. Polished stone slabs cannot be used in crafting however.
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EMC Exploit[]
It takes three Stone to create six Stone Slabs. Both of these items are worth one EMC, therefore three EMC makes six EMC. You can automatically double the EMC coming from a Quarry by smelting Cobblestone using an Induction Furnace and crafting the resulting Stone into Slabs using either of the Automatic Crafting Tables. It's not very much EMC per block but with a 64 x 64 Quarry roughly 200,000 free EMC is available.
Item Exploit[]
Making Stone Slabs also doubles the number of items, from three to six. This is good for Recyclers where it takes a lot of items to produce few Scrap. Although there is a 1 in 8 chance of generating Scrap, doubling the items going into the Recycler, by turning Stone into Slabs, increases the chance to 1 in 4.