I started to remove the old caulk and found it is hard and grainy

Get out your propane torch and see if a little sample will burn. Most grouts
are low in organic material content and will probably not ignite. Silicones
abused in flame will leave a white ash behind. Testing reaction to a paint
remover with methylene chloride content might be informative. A grout would
probably show littlle effect.

I'll leave the grout vs. caulk question to more experienced tile experts. HTH

Joe