I personally believe that dairy is a safer source of calcium than ground seashells. When arteries get clogged, it’s more calcium than anything else. Pig fat and beef fat act as ‘glue’ that collects any calcium that is ‘sharp edged’.
Milk has calcium from grass that is much tinier and less likely to clog arteries.
Milk and dairy, tree collards, sesame seeds etc. There are many sources of calcium in food, and I agree that they are better than supplements/ground shells.