Hi. I did some searches and did some engraving on a river rock that turned out ok but will try again with a thicker font. I used a thin one and was hard to read. I had read some rocks were not ok to use. I do not know what these are so checking if anybody knows what they may be and if ok to put in the glowforge. Thank you in advance!
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Lovely work - and at least in the picture it’s totally readable 
AFAIK there isn’t a rock out there that will hurt your 
, but there are some that won’t engrave well. Some may not cut, others will crack, yet others may craze. Your best bet is to test. A lot of times those mixed bags of polished rocks are dyed quartz - other times they’re different gemstones.
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If you want the engraving to show up better, you could always wrap the rock with masking tape before engraving, then paint the area before peeling the masking off.
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Thank you for the replies. I did try all 4 of them. 3 came out ok. Some chipping. The white is what it came out like, no paint. The dark one chipped quite a bit and was hot when I took it out which scared me.
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In the past when I engraved tile, common rock, or slate I would see glowing sparks of material being ejected at high speed straight up from the surface. I do not know if it is possible to chip the laser lens. A sand blasting effect on a car windshield? Just a personal concern of mine, not something that I have noticed.