What is the nature of cell-walls in diatoms?

Explanation

1) In Diatoms, the cell-walls form two thin overlapping shells. The upper one is called epitheca and the lower one is called hypotheca. They both fit together as in a soap box.
2) The walls are embedded with silica.
3) They pile up at the bottom of water reservoirs and form big heaps called diatomite or diatomaceous earth.