What was this object at the end of an episode of The Flash?

Question

Potential Spoilers Ahead!

At the end of Season 3 Episode 21, Team Flash has successfully

built the Speed Force cannon bazooka.

However, a couple of them mention that it will need a huge power source to work. As the show fades out, Barry looks (wistfully? knowingly?) off to the side and the last view that watchers get is of:

a reddish, glowing stone in some form of container, next to a menacing King Shark.

What is this, and was Barry's aforementioned look an indication to viewers that he was thinking of it? I can't recall if this is the first appearance on the show or if my memory of previous episodes is failing me. I have not seen the most recent episode of The Flash: SE3E22, Infantino Street.

Answer 1

The red object shown at the end of S03E21 is some sort of technology/power source from a Dominator ship acquired during the crossover event earlier this season.

Lyla explains:

It's a piece of the Dominators' technology. We got it from the ship that crashed in Central City.