What is the nature of cellwall is diatoms?

(a) In diatoms the cell walls from two thin overlapping shells which fit together as in a soap box.
(b) These walls are embedded with silica forming siliceous frustules which are indestructible.
© They pile up at the bottom of water reservoirs and form big heaps called diatomite.
(d) Diatomit is used in filtration of sugars, alcohol and antibiotics.