I have personally spent around 5 real life hours in own world also sprinting non-stop searching for any Limestone or chalk to no avail. Numerous chunks of fully explored rock layers. I mean everytime I found a new rock on the service or through a cave I ALWAYS checked down far enough for 4 different rock layers and for them to loop back around. Thousands of blocks ran in every cardinal direction and nothing. The only lime I can get is very limited from 1 trader and ive found a total of 5 shells which is nothing. I have a rough time believing this is just simply "bad luck".