Why does engraving a rock leave raised lines instead of carved ones?

What happened is the surface of that pebble melted into glass. Smooth river rocks are mostly silica, and when silica gets hit with enough heat it liquefies, then freezes into a shiny bead as it cools. You basically made tiny lava. Slate acts completely different because it's layered and gritty, so it powders out and blows away, leaving a matte recessed mark instead of a glazed one. Same machine, opposite result, all down to the rock. For a sunken mark, the trick is less heat per pass, not more. Drop the power and raise the speed, then go over the same line several times lightly. One slow heavy pass melts everything, but a bunch of quick shallow passes mostly ablate and the debris gets blown out. Also wipe the stone down with alcohol first, yard rocks carry a film of dirt and oil that changes how the beam interacts. And run a test grid in a hidden spot, small squares at different power and speed combos, until you find the point where it stops glazing. On smooth silica stones you might never fully avoid some glassiness, but you can minimize it. On the Braille idea, honest answer: the raised beads are probably not reliable enough for daily use. Single pass ridges are way too shallow and uneven to read by fingertip, and dots built up from stacked passes are glass, which means brittle, sharp edges and a real chance of chipping off. Real Braille dots have a consistent rounded shape and a specific height and spacing, and a laser bead is going to vary dot to dot.

If I were making that gift, I'd engrave shallow round recesses where the dots go and then fill them with tiny drops of epoxy or UV resin, sitting slightly proud of the surface. Once cured they're smooth, rounded, tough, and they stay put. Do a test pebble and rub it between your fingers a few dozen times, check for rough edges or anything lifting. That way she gets something that reads cleanly and doesn't fall apart in a week.

That matches what I saw on a beach stone I tried last summer, glassy bead every pass no matter what I dialed it back to. I ended up switching to a rough concrete paver for testing and the marks went clean and sunken right away, so the rock type really is everything here. For the gift, the resin dot idea is the way I'd go too. I did something similar on a garden marker using two-part epoxy dropped into shallow dimples, and after a year outside the dots are still smooth and holding. One tip for the fill step, put a bit of masking tape around each dimple so any overflow wipes off the stone easy once it sets.

Soapstone is worth a look for the sunken version. I engraved a soapstone trivet for a neighbor and it cuts a clean matte recess like butter, no glaze at all, at 80 percent power around 250 mm/s on my 60W unit. Big-box stores sell plain soapstone tiles for a few bucks, way more predictable than whatever silica mix is in a yard pebble.

On the readability side, dot spacing matters as much as dot height. Standard Braille cells are tightly spaced, so if you're placing dots by eye they'll be too far apart to read as words. Print an oversized Braille template on paper, lay it on the stone, and mark through it so the layout is actually right before you commit to the resin.