The calcium in egg shells is calcium carbonate, also known as Tums. It is a significant acid reducer and that is not a good thing. That is why too much can cause an upset stomach and is just one reason why carbonate form is less than ideal.
Why would I recommend dosing silica? Largely because creatures in our tanks use it, the concentrations in our tanks (at least in mine) are below natural levels, and the sponges, mollusks, and diatoms may not be getting enough to thrive.
Bauxite is a result of intense leaching in a hot and humid climate with alternating wet and dry seasons and good downward drainage. The climate promotes vegetation that provides organic acids, which help to dissolve rocks in percolating water that carries more soluble components away, leaving only aluminum and also often iron as the least mobile common ions behind.