birthday gift for bmom

we are going to visit with my son's bmom a couple days after her b-day. i wanted to get a necklace for her with our son's birthstone in it, while at the store, my son picked out a very pretty silver heart locket. of course i had to buy that since he picked it out. the problem is the heart is so small, i just can't shrink a picture small enough to fit in it. i was going to put a very small lock of his hair in there. do you think this is gross? or have a better idea?