Board Thread:The Flash discussions/@comment-9346948-20170510001536/@comment-26266279-20170514152835

Gracekim12 wrote: Kaestal wrote: Still waiting on that Philosopher's stone explanation I'm sure they'll explain it at last (Hopefully during tuesday's episode)

Yea I am hoping the same along with the box it was contained in.

The stone is  linked to Savitar's trap inside the speedforce (seeing as Wally got swapped with Savitar when he threw it into the Speedforce) and Savitar could manifest outside the speedforce only when the stone was exposed outside the box.