nostr:npub1lv4hccv9t9y0wall78lkfamydwcyepmyxx05c8mkpk2lqnfyzcxshuru5g
I guess this isn't a horse chestnut?  Inside is a shiny brown 'conker' that kids (and not just kids)  collect. 
But I'm sure you know conkers and horse chestnuts. So this is probably not what it is.  (The little flowers are gorgeous, though not often noticed.)