You want to stay away from copper and from chrome plating because they are reflective to the wavelength of the laser. Some people think that a reflection would be too diffuse to do any damage to the inside of your laser. Personally, I don’t see any benefit that would outweigh the risk. Chrome-plating is not super expensive, but it is also not cheap, so while I doubt many mirrors would use it you may want to verify.
Anything with chlorine in it should be avoided because the gas released is very, very corrosive to the insides of your glowforge. PVC (think the white tubing used in plumbing and vinyl) is the biggest no-no, but if you think it may contain chlorine best to not try. Also, the gas is bad for you and your lungs, but I’m not sure how you’d get a good whiff of it in a properly ventilated setup.
The gas released from cutting chrome-tanned (also could be referred to as oil tanned) leather may contain small amounts of hexavalent chromium a known carcinogen. It would appear that the amounts released from normal cutting would be considered safe by OSHA for an adult, and if vented there should be no risk, however, it is perfectly understandable to play it extra safe with your newborn. Please note this does not apply to leather that has been vegetable tanned.
Now on to fun stuff. Yes, you can get deep engraves in stone, but other stones are barely marked. Travertine tiles, pretty cheap at places like Home Depot/Lowes, can be engraved pretty deeply. I did some and found if you go too deep after a few weeks a talc-like, slow motion eruption may occur and crack the tile. But that was pretty deep. It has been 4.5 months since I made some coasters with a just right depth of engrave and they have been fine. As for other metals, some mark pretty well and others are pretty resistant to change. You just have to try and see. One thing I have read here, is that for anodized aluminum you have to be very precise in the measurement you enter in the focus height for custom material to achieve the best results.