Eggshells are a great source for three important minerals calcium, phosphorus, and magnesium. 
 
These minerals are key to strong bones and teeth and your dog’s overall health. But that’s not the only role calcium plays in your dog’s body. It also helps with blood clotting, muscle contraction, milk production in lactating dogs, heart pumping, vision, and healthy metabolism. They are essential for your dog’s blood coagulation, muscle growth, and nervous system function. Luckily, eggshells contain plenty of Calcium and Phosphorus.
 
Eggshells also have plenty of magnesium, which is another mineral that your dog needs for good health. Dogs need magnesium to properly absorb calcium. It also plays an important role in muscle and bone development.
 
Eggshell is best given in powdered form because the powder is easily added to food and allows your dog’s digestive tract the opportunity to absorb minerals from it. It's also an effective alternative to raw bones or bone meal.
 
Eggshells are roughly 40% calcium, with each gram providing 381–401 mg of Calcium