I haven't seen window sills made of stone here. but the idea is nice. it's solid, stiff and resistant to water and any dirt. ideal for growing plants on window.
banana has too big leaves. I believe it needs a lot of heat and light. I grow citruses at home for many years. also some other plants, maybe 30-40 plants totally. I didn't count them. I do it for years, since childhood. my grandma was an agronomist and she knew everything about plants. and I was grown up by grandma and since childhood I was involved in all that gardening works and thing. and I know a lot about plants too. and I like to tinker with plants, soil and all that. Icould work as a gardener, if this was any paid. but it's not demanded over here. and I don't have any plot of soil I own so I grow plants at home.
over here the climate is too severe for successful gardaning. some plants grow well, but not many. severe long winters (we have snow half of a year) and rainy short summers. we have continental (and close to polar) climate zone and 2 or 3 in plants hardening zones scale. it's called the "zone of risky agriculture". so such things as citruses grow only at home. tomatoes, cucumbers and such grow in greenhouses in summer. there're plants that are resistant to cold sorts of plants, but not many.
stone made window sill or just looks like stone?