Peakstone Slabs were introduced with Patch R36 on November 15th 2016.
2 of these slabs can be made in a Processor from one block of Peakstone each. No crafting recipe has to be unlocked for that.
Peakstone can be found high up on Mountains, most often snowy ones. Rambeaus often spawn on Peakstone during daytime.
Obtaining Peakstone Slabs is the one requirement necessary to unlock the crafting recipe for Peakstone Stairs.
Peakstone Slabs are also one of the requireded ingredients to craft Peakstone Stairs.