Usually when wood is labelled treated eg treated pine, it means it's impregnated with arsenic and copper to protect against insects and fungi. It starts off green and weathers to grey. That kind of wood is only used for building, fences and similar, not for furniture because it's not good for us either.
A rattan chair is probably lacquered, but that is not the same as treated.
Lacquered rattan is a bit orange and a bit shiny. If the chair is old you'll probably see bit flaking off where there is wear and tear.