"Fait de pierres" suggests that it is made of an assembly of cut out pieces of rock.
If it were made of stone as a material (vs steel or concrete), the French sentence would be "le pont est en pierre", in singular.
Also "fait(e)(s) de + noun" drops the article, in singular or plural, countable or not: fait de soie (silk, fem), fait de petits morceaux (small bits), fait d'or et d'argent (gold and silver, masc).
There are several ways to say this. The only tricky part is whether you want to focus on the plural word "pierres". In English, one could, but would probably not say "made of stones", but "made of stone". As it turns out, you can say the same thing in French, i.e., "fait de pierres" or "fait de pierre" (also "est en pierre", or "fait à partir de pierre").
I think that there should be an option to include/exclude writing in lessons. I personally don't mind if it has a few problems or inconsistencies. Some people seem to get really upset about having "correct answers" marked wrong, but I feel like I'm missing out on a good way to retain what I learned (including accents, spelling, etc.)