Eph 2:19- you are fellow citizens with the saints and members of the household of God, 20 built on the foundation of the apostles and prophets, Christ Jesus himself being the cornerstone, 21 in whom the whole structure, being joined together, grows into a holy temple in the Lord
Is Jesus a stone? The whole analogy is a metaphor for his centrality to all other foundations already laid. When they were laid is irrelevant as Jesus place in the foundation was already known.
1 Pet 1:20 having been foreknown indeed before the foundation of the world, but having been revealed in the last times for the sake of you
What exactly is the 'cornerstone' that Mark 12:10 speaks of?
Other verses answer this quite succinctly, Jesus IS the cornerstone and that is all we need to be mindful of. All other stones must align with the cornerstone. Obviously, this is a spiritual concept expressed through a building foundation metaphor. Whether there are other stones on other corners is moot. Jesus is the author of salvation, which is what the temple symbolises.
Acts 4:10- This Jesus is the stone that was rejected by you, the builders, which has become the cornerstone.