Using Magneto (NOW!)'s purple power, I created a U-shaped pattern, i.e.
B Y B
B Y B
B B B
where ‘B’ is a blue gem and ‘Y’ is any other gem.
In my case, only the two vertical sets of 3 were recognized as a match. I’m not entirely sure what I was expecting to happen, but I did feel a bit unfortunate not getting even a single critical tile out of the thing.