The Singing Stones of Hamlin are a naturally occurring crystal, which hums with a barely audible song. When that tune is hummed, it has potent healing magic.
They each have 12 charges.
For a free action and one charge, you can hum it's resonance and get the equivalent of a healing word, 2d4+4, first level for you or a party member or ally.
For 2 charges and a bonus action, you get a 1st level cure wounds, 2d8+4.
For 3 charges and an action, you can get a 3rd level cure wounds, 6d8+4.
And after battle, you can do a ritual with multiple stones that will cost 4 charges for every stone involved, 3 minimum. That will give you each 8d8+14 healing.
For 5 charges from a minimum of 3 stones, you can cast revivify.
You can be attuned to only one stone at a time.