Intriguing... With the stones in mind, are they all the same kind of stone? Or do some have diamond hearts while others have hearts of malachite, ruby, sapphire or onyx? Is it only gemstones or any kind of geological formation? Thinking that different types of stone could influence character and other aspects of the elf it belongs to. Could also fix the problem of people hunting all elves since some would be worth less or maybe even worthless to anyone who isn't interested in necromancy or other kinds of magic. So you wouldn't risk hunting an elf unless you knew the quality of the heartstone, which would take considerable research and stake-outs to discern, since personality based on stone is a rough approximation. Maybe?
Currently elven heart stones are rubies and indeed legend says that elves originally fell from ”the wandering red star”. There might however be a few deviants, both in stone type and temperament …
It seems that younger elves start with ruby shards that grow over the years like pearls in a clam, so that the oldest ones are the biggest and most valuable. This is in line with what Harper suggested I believe, just didn't think of it. If you look at the map of Ravenland, there's an island called Maidenholm in the upper right corner. According to legend, the elven maiden in question gave here life and split her heartstone to give birth to several younger elves that still are around.
Like you imply, elven heart stones are used in necromancy, demonology and alchemy or simply sacrificed to various gods by being smashed on an altar. There isn't an economy to support organized trade though. The only ones willing and able to pay for a genuine, mature elven ruby are rich noblemen and even then they have to take into account being targeted by elven retaliators if the don't keep the treasure a strict secret.