Can I feed my gerbils egg shells to add calcium to their diet?

I like to suppliment my gerbils musli diet and have discovered that so far they like carrot, broccoli, dried dates, hazel nuts, sunflower seeds, walnuts, pumpkin seeds, dried parsnip, dried beetroot, blueberries, bran flakes, and of course gerbil treats such as calcium bites and apple treat-ums. I know i can also try bread, other veg, fruit, cereals, seeds and nuts, cat biscuits, cheese and egg. What else can I give them? I know they need lots of calcuim to grow their teeth, what about egg shells?