No problem Aussie, the mistake was understandable
Paleoarts, thanks for your help, that improvised rock chisel sounds interesting. I wonder would flint be hard enough to chisel out grooves in the kind of rocks I'm using though (rounded basalt/granite type rocks from the beach), or would it just lose its edge in no time? The ancients probably used slightly softer rocks, sacrificing a bit of hardness/density for easier workability...
I see you mentioned a modern steel rock chisel, that'd work too I suppose? I presume you can get these at hardware stores, I'm not just up for investing in a tile saw just yet, that's one serious power tool