Lost a heart because the translation means both "in the garden" and "in the yard". I thought the yard was "el patio" - but what do I know!
Both "el jardín" and "el patio" can be used for "the yard." It depends on which country you are in as to who uses which one more.
"There is" is for singular = there is a stone in the garden.
"There are" is the correct answer for plural (stones)