Digestible bones (tinned Sardines and Salmon) are a great source of calcium. Soaking dried corn kernels in lime water makes them absorb calcium. This is sheer nature magic and wisdom combined.
In Mexico, corn tortilla is the most widely consumed staple food. The corn dough used for tortilla preparation is traditionally prepared by boiling corn in a lime solution that contains calcium, and letting it stand overnight. This process is called nixtamalización.
In addition to calcium content, calcium absorption from food sources is also determined by food composition. Food components such as oxalate, phytate, fiber, lactose, and protein, affect the efficiency of calcium absorption. In green leafy vegetables, almost all of the calcium is bound to oxalate. Studies showed that the absorption of calcium from food sources such as spinach is very poor because of the high content of oxalic acid Higher bioavailability of calcium from the diet was shown to improve bone health
REF: Calcium Absorption from Corn Tortilla Is Relatively High and Is Dependent upon Calcium Content and Liming in Mexican Women, Jorge L. Rosado et al, http://jn.nutrition.org/content/135/11/2578.long