Best Concrete for Pet Headstone
You're a metal worker? How about stainless? 

This DiY Headstone suggests high strength mortar mix.

You should use colorant to achieve your final color instead of attempting to stain it afterwards, which would only be skin deep.



However, this is why you use granite: 



I'd suggest Uba Tuba for its flakes:



Truly though, what I'd recommend, is having a real stone cut for you. They're not as expensive as I would've thought. Even a somewhat fancy one should be under a grand. But it doesn't really matter what we think... what does The Boss want?

This:



Or this:



Food for thought, regarding the fact that you're a metalsmith, if your only tool is a hammer... 

If it needs to last, then it needs to be made out of granite.