For the fun of the experiment I tried freshly sqeeuzed lemon juice... nothing happened.
Then I googled and I found that I could also use normal vinegar.
And as you sugeested used small medicine tube for letting one drop fall.
And looking through a magnifying glass I saw little bubbles show up; not vigourously but they were there.
That was fun and I did not know that I could use vinegar!
So... it is a calcite then?