where did the rock come from in stranger things

The “mysterious rock” in Stranger Things is presented as an otherworldly artifact whose exact origin is not fully explained on-screen yet, but recent official comments and tie-in material strongly suggest it comes from outside our world and is directly tied to the Mind Flayer and the Upside Down.

What the show and creators suggest

• In the Season 5 finale, the rock is shown as a strange, fractured-looking object whose patterns resemble the ground of the Upside Down, hinting that it is a piece or fragment of that dimension or something closely connected to it.

• The Duffer Brothers have described it as an extraterrestrial artifact that first exposed Henry Creel (Vecna) to the Mind Flayer, setting up his transformation and the birth of the Upside Down threat as viewers know it.

How it got to Henry / our world

• The rock is shown in a briefcase in human hands, implying it was transported or smuggled into the human world rather than naturally occurring on Earth.

• Fan discussions and some ancillary material indicate it was stolen from an experimental program (hinted to be connected to Cold War–era research, including Russian involvement), then ended up in the possession of the man carrying the briefcase, which is how it eventually “reaches” Henry.

What is confirmed vs. still mystery

• Confirmed or strongly implied:
  • It is not just a normal meteorite or Earth rock; it is treated as an alien or interdimensional artifact.
  • It is the catalyst connecting Henry Creel to the Mind Flayer and thus a key piece of the Upside Down’s origin story.

• Still open / for the spinoff to answer:
  • Exactly where in the cosmos or multiverse the rock originally formed.
  • Who first discovered it, how long governments or secret programs have known about it, and whether similar artifacts exist elsewhere.

Role in upcoming spinoff

• The creators have teased that the spinoff will dig into the rock’s background, including:
  • The identity and story of the man who had the briefcase.
  • The true origin of the rock and its connection to the Mind Flayer and the Upside Down.

So, as of now, the best concise answer is: the rock is an alien/interdimensional artifact linked to the Upside Down and the Mind Flayer, brought into the human world through secret experimentation and ending up with Henry Creel, with its full backstory intentionally saved for the spinoff.