Petrified
One of your blocks has turned to stone. You can't swap it, but you can still match it by working the blocks around it, and a match right beside it cracks it free. Clear below it and the stone drops like anything else. Left alone it crumbles away by itself.
In detail
A petrified block can still be matched but cannot be swapped, and it falls with gravity like any other block. A match orthogonally beside it cracks it free, and it also crumbles on its own after 30 seconds. Jotunheim's signature status: Crag Jotun and Stone Sentinel land it on a connecting strike, and the Stone Wyrm's Petrify Breath sweeps top to bottom and briefly petrifies the whole grid, with a few cells lingering.
More in glossary
From the current development build. Content and balance may change before release. This public reference includes discoveries and does not reflect your personal unlocks.