Go to a hardware store and get the size bit you need that is made for metal. Most cast iron isn't really hard unless it is white cast iron. If the hole is over 3/16, pre-drill it with a small bit first. It might be a good idea to use some cutting oil to.
For more info, see the section on calcium absorption. High calcium foods include dark leafy greens, cheese, low-fat milk and yogurt, bok choy, fortified tofu, okra, broccoli, green beans, almonds, and fish canned with their bones. The daily value (DV) for calcium is 1000mg.
Milk, yogurt, and cheese are the main food sources of calcium for the majority of people. Milk also has added vitamin D, which is important for helping your body better absorb calcium. Calcium can also be found in leafy, green vegetables. Kale, broccoli, and Chinese cabbage are good vegetable sources of calcium.