Adora Calcium
Never heard of the kidney stone thing so I can't confirm that, but the calcium citrate is supposed to be more readily absorbed. The carbonate requires an acidic environment which we don't provide anymore. Supposedly, the citrate is easier absorbed by everybody, not just post WLS patients, but it's also more expensive.
Edited to add: Looking at the site, it's calcium carbonate
Edited to add: Looking at the site, it's calcium carbonate