It fracures like slate(sedimentary), most use it for farm tracks and cheap infill, I think the brown in it is Iron..I’ll see if I can find the actual name.
Also tried some greyish sandstone…but no marking resulted.
It would probably be nice to have a degree in geology to tell these apart.
I did have a couple of pieces of slate that apparently had some other component makeup.
This is a heart in hands graphic.
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Curious as I am, I didn’t want to miss out on trying this laser variant (engraving on stone), so I started with slate, which seemed “too easy” to me.
I found some 10 x 10 cm (4 x 4 inches approx.) slate coasters in one of those supermarkets that sell a little bit of everything and bought a box of 8 for €5.
A little creativity with Lightburn, and voilá.
Now, a question, if it’s not off-topic: To prevent any liquid that accidentally spills (or consequently 
) from being absorbed by the slate, what kind of treatment do you suggest? Something “food-grade,” I believe, that allows for hand washing.
I don’t coat mine with anything. In my opinion, a coaster should absorb small amounts of liquid / condensation that gets on it. This protects the surface it’s meant to protect.
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Not off topic here. You lasered some rock, so I think it qualifies. 
You’re right about that. That’s what distinguishes a purely decorative object from a useful one. Good point!
Thank you.
My concern is that the opinions on types of preventive treatments might stray from the core of the topic. 
I agree with the idea that coasters are best when they can be at least a bit absorbent. With slate coasters, specifically, with some settings on some of my lasers (I don’t have a problem!), the surface texture of the engraved area gives good contrast while dry but poor contrast while wet. In those cases, surface treatments can be undesirable if they reduce the contrast to the “wet” look.
That’s not a problem if I’m doing slate coasters on my MOPA fiber laser, on the other hand. I do a two-step process on those where first I do a deep engrave and then have a second fill that turns the engraved area into a vitrified micro-pebbly-ish surface. That laser-treated surface doesn’t darken at all when wet, so wet contrast is actually better than dry. (I don’t know whether such a concept would be appropriate for lasers that only mark, not carve into, the surface.)
Deep engrave: 4000mm/s, 70% of 60W, 40kHz, 200ns, 0.01mm, bidirectional, cross-hatch, a handful of passes based on desired depth.
Treatment pass: 1000mm/s, 70% of 60W, 400kHz, 200ns, 0.05mm, not bidi, not cross-hatch, 3 passes.
I need to play around with some rocks with my 30W CO2 galvo, just to see what that does. (I believe I said up thread I didn’t expect to get one of those, but oh, my, does it ever make things like cork coasters fast. Alas, cork’s not rock, so that’s a tale for another thread.)
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I am pretty sure the “core” here is what happens to rock material when hit with a laser. @ClayJar squeaked by when he said…
Really, I think as long as the material is about some sort of stone, it is On Topic.
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Sounds like you’re melting the stone. I have to keep my fiber pulse rate low or I have a tendency to melt the rock. Lava rock is really difficult.
Absolutely. Going too hard on the settings will turn slate into black bubbly obsidian-like glass that is rather undesirable. With just enough, on the other hand, you can do just the minimum amount of melting at the surface. It doesn’t change the color much (it goes from the engraved-white to ever so slightly slightly tan). On the other hand, instead of a microscopically-rough surface that goes dark when wet, you have a microscopically glassy surface with the light-scattering texture inside which therefore doesn’t darken at all when wet. (It actually looks better when you wet the coaster.)
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My only laser machine is a 5W (optical) diode laser 450nm so no kHz or ns to set but I will give it a try.
After this coasters I am felling tented to make a cheese / ham slate board or some kind of “welcome sign” to hang on the wall but in this case I think it is better give some treatment after.
And I think even I can “melt” the slate on my diode laser I will have to wait for the rest of my life that the slate is finished. 
So, I am officially looking now for treatments to preserve this “fried rocks”.
Food-grade and non food-grade.
I think it might be one “plus” to add to this topic, right?
My laser is only 10w, so you should be able to Fry Some Rocks too. It did not take me very long to do the first one. Just pay attention to the postings about rock dust.
Food grade or not? Unless somebody is going to be eating them, I don’t think you need to worry about that. 
Figured as long as we’re rocking out, I should try something on my 30W CO2 galvo. I mean, yeah, I generally do rocks on the 60W MOPA fiber laser, but mine is an “Eh, why not?” shop (except for safety, of course).
So, an unpolished black landscaping rock from a bag from Home Depot, hitting it hard enough to melt the top layer and get a nice dark obsidian. Settings not at all optimized and results not dialed in, but hey, it’s a whynot, after all.
Speed: 1500 mm/s
Power: 100% of 30W CO2 galvo
Frequency: 200 kHz
Line Interval: 0.0250 mm
Fill individually, one pass.
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I really love that sparkle effect in the burn! Too bad we can’t order rocks of one type instead of burning 10 and getting 2-3 looking the same.
Have you noticed that even the sides get burned although those are way out of focus?
Had a thought… To help keep down the rock dust, I am going to try spraying one with a water based clear urethane (Varithane). It will be in a couple of weeks, away from home and all my toys, now.
For the bags of black landscaping rocks, the Home Depot unpolished ones are generally consistent. The Lowe’s polished ones were the first bag I bought, but the current bag seems to have two varieties, one of which gets splotchy. The bagged colorful landscaping rocks tend to have significant differences in how they react, and there are a good handful of different types in there. Too few of the variants make me really happy with their results, but I’ve still been playing with them a bit here and there.
As for the focus and such, I go with a medium lens when I’m doing rocks. On my MOPA fiber, I have 75, 175, and 300mm field sizes available from my lenses, and I go with the 175. On the CO2 galvo, I have a lenses with 110, 210, 300, and 400mm field sizes (which may be just a bit less in real life after I swapped from a 3x to a 6x beam expander and replaced the galvo head). This last rock was with the 210.
Generally speaking, I’ll aim for an area on the target rock where the engraving is as close to planar as reasonably achievable and just let the depth of focus take up the slack. Usually, there’s not too much tail off where the surface curves away, although if it looks like it might be borderline, I’ll drop focus a bit below normal to split the difference between the low and high spots.
(Also, using a sandbox for fixturing is the best thing ever for engraving rocks. It’s just a leftover storage box with some dry play sand, but it lets you quickly and easily set a rock in any orientation you want. You do have to redo your focal distance for each one, but they’re rocks of arbitrary size and shape, so you’d have to anyway.)
As for rock dust, I don’t know if it’s my extraction system, materials, lasers, settings, sandbox, or whatever, but I haven’t run into anything notable with rock dust. I don’t think a coating would help with that on the fiber laser, regardless, as it would just be additional dust. On the other hand, I have used simple spray lacquer to give rocks and such the “wet look” on the unengraved areas, and that’s been fine.
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Thanks for bringing this up.
I have the machine enclosed in a small awning (a small enclosed balcony). Whenever it’s working, I close the glass door and monitor the machine to keep me away from the fumes, dust, and other chemicals released when the laser operates.
That doesn’t mean there’s still something in the air when I access the machine after finishing a job…
In any case, I’ll be more mindful of the stone dust and clean the machine more frequently.
For me, “what doesn’t kill you makes you fat.” More or less the same as “what doesn’t kill you makes you stronger,” but from a nutritional perspective.
But here in Portugal, a country on the European continent where society has become petty when it suits it.
If I present something that’s supposed to come into contact with food and isn’t properly protected, I’m an irresponsible and incompetent person who doesn’t consider the health and lives of others.
(But then they’re capable of eating anything as long as it’s heavily seasoned to neutralize its flavor, and perhaps even something much more harmful to their health.)
Sooooo… having some knowledge about food-grade protection matters.
If you have an effective extraction system, you won’t see any dust, especially if it’s a small area.
I believe you’ll notice some dust in a machine without extraction (which is the case with mine) or after a few hours of work, or over a large area.
I smelled something in the air on these coasters but didn’t see anything in the machine, although we’re talking about dust, most likely microscopic.
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By chance, I managed to get a sample slate (600x300) 7-9mm, euro’s 3.95 each from a farm supply outlet and was keen to try it out.
Not enough time today to do a lot of experiments but I was very suprised by the range of colour tones and a very smooth to gritty sandpaper finish in the higher power range.
I’ve been studying really hard concerning images on wood and photo editing software, lightburn, laptop photo edit and gimp for about three weeks and the better I got at it (very slow process) the more it felt like I was just competing with photography…thats a loosing battle from a time and cost perspective.
I thought the only thing I can do is to do with an image what a camera cant, so I’m quite pleased with the initial tests and looking forward..ish to the Much experimentation ahead with slate.
Forgot to mention!
Snapmaker Ray, 20w
Lightburn material test grid and circular tests as used by Gil..‘youtube’ “Thanks Gil”.
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Here’s a rock I did yesterday. I bought a bag of 10 assorted rocks off Amazon and find they all burn differently. Some of them just vitrify rather than being engraved. This is done with my 60w Jpt fiber laser using a 220mm lens.
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Nice! I discovered the same thing with my Amazon order.