I got some random rocks from Amazon (for painting, according to the listing). Most of them have not taken lasering too well on the fiber laser. The settings I use for slate or the black landscaping rocks haven’t really done well. The BREC Outdoor Adventure test rock in the second image is a typical example – it’s kind of “foamed up” in the engraved areas.
So, naturally, I took one of the reddish ones and tried a full-parameter-space search. (It would’ve been really convenient to have logarithmic steps in the Material Test, so I’ve got a thread on that now, heh.)
Anyway, with very low speeds, I can turn it into dark obsidian-like bubbly glass, and with higher speeds and other parameters, I can get various shades of brightening. Anything with too much heating turns flaky with little bits popping off (and you can see in the center of the first rock how the entire surface near the black glassy spots completely peeled off in one big flake). I can get a decently bright result by staying just under the flaky limit (using all four parameters on my MOPA fiber), and the inclusions tend to go black.
The Ward Fontaine rock was done with my best-seeming parameters for that particular type of reddish salami rock. I hit it with the settings, then tried a couple more times, checking between each run. Didn’t really change the brightness, and didn’t have any notable ablation/engraving, so once through is likely the ticket for this particular rock. It’d probably be a nice rock for clipart-type graphics with moderate to low detail, but definitely not the best for text or fine detail.
