So after playing for awhile and acquiring things...
Flasks and vials stack to 99, and beers, milks and fruit juices, all stack to 99, and essences, oils (enchanting, tanning acid, tanning oil) all stack to 99.
Potions (bulls Str, clarity, and the like, not the healing potions) do not, they only stack to 10
Nor do alchemy things like holy water, acid flasks, tangle traps, and fire flasks... they only stack to 10 as well.
Other things that stack are parchments and scrolls to 99, but sandpaper does not, it only stacks to 10.
And last but not least, raw quartz and raw onyx don't stack at all, but all other gems in all states (raw, cut, fine, and dusts for that matter) all stack to 99. Can we make those two stack as well?