Did a settings test about two weeks ago, left it on my work bench and came back to this.
Chemical reaction to the air? Think immediate marble sealant would help? Only occured on the 100 speed column. Hoping the rest doesn’t slowly dissolve because while the science geek in me thinks this is wildly cool, the designer half worries. Anyone else have this happen?
Looks to me like someone came around, swept up some marble dust, and dumped it on the engrave test. 
You created quicklime. Lasing hot enough you turned CaCO3 into CaO and CO2. CaO is highly reactive and grabs water out of the air and CaO + H2O → Ca(OH)2 that is Calcium hydroxide that pulls more water out of the air and of course is much more physical volume.
And sealing it won’t help. The stone is likely too porous, so humidity will get in no matter what. I think you may need to find different stone.
Wow, my travertine marble coasters have held up over a year at this point with none of that
Can I just say right now how much I love science geeks. 
So a quick run will not become quick lime, and washing in water will take care of what does.
Science!! 
what a cool visual of the reaction
I learn so much here. 
Thank you for the explanation! Really, really neat.
My first experiments with travertine I cut deep and a couple days later it looked like a miniature science fair volcano. Shallow engraves are fine two years later.
It would be my guess that soaking in ice water might short circuit the volcano problem but keep in mind that the reaction is very exothermic if there is enough CaO. but after the reaction is over all should be good.
