Found some basalt tiles at HD, testing on some mineral-oil covered stuff yielded the following at 225 LPI. First two columns are 1 pass (905s 30p is at the top and 600 30 at the bottom), and rightmost column is 2 passes.
This is a “good” shot with a good reflective angle on the light.
This is a “bad” shot looking straight at the material.
I’m probably going to write this one off as an engraving material (still would work as the base for a tape stencil as it’s very flat, albeit porus). Without the mineral oil (and really even with it) there’s basically no visual contrast.
Just got my glowforge a few days ago and I’m already burning through plywood - love how easy this machine is to set up and use (I’ve had an X-Carve for a long time and there’s nothing I hate more than zeroing the bit and removing tabs).