The short answer
On slate the engraved area is the light area, so the image logic is inverted compared with wood. Prepare the photo for a short tonal range with a clear subject-background separation, let the untouched stone carry the darkest parts, and test on the same tile you will use, because natural stone varies. A dither pattern usually beats a greyscale ramp here, because the surface either lightens or does not.
Engraved = light, untouched stone = dark
Short tonal range: aim for clear separation
Test on the same tile, not a different one
The inversion, explained
On wood you burn the dark parts of the picture. On slate you engrave the light parts. Software handles this with an invert switch, but the consequence is easy to miss: a photo with a large white background becomes a large fully engraved area — the whole background gets frosted, which is rarely what anyone wants on a slate coaster. Either remove the background first or plan for it deliberately, for example as a framed panel.
Invert changes which areas get engraved
A white background becomes a fully frosted panel
Remove or shape the background before you engrave
Slate is a natural material
Two tiles from the same box can differ in surface tone, hardness and coating residue, and a single tile can vary from edge to centre. That is why a settings chart from another workshop is a weak starting point here. Clean the surface first, run a small test patch on the same tile, and note that many slates come with a protective coating that behaves differently from the stone underneath.
Tone and hardness vary per tile and across a tile
Clean before engraving; residue changes the result
Test patch on the same tile, then commit
How Vextrify helps
Vextrify carries a slate profile with the inversion already handled, so the preparation works in the direction the stone needs rather than leaving you to reason about negatives. Areas that are white in your source image stay untouched stone instead of turning into a fully frosted panel, and the camera view lets you place the picture exactly on a tile that is rarely perfectly square. The browser version is free and needs no account.
Slate profile with the inversion handled
White in the source stays untouched stone
Camera placement for a tile that is not square