Statement (I) is true: Diatoms belong to the group Chrysophytes. Their cell walls are unique "frustules" composed of two thin overlapping shells that fit together like a soap box. These walls are heavily impregnated with silica, which makes them extremely hard and resistant to decomposition, often described in biological texts as indestructible.

Statement (II) is true: Because of their silica-based walls, diatoms do not decay easily after death. Over billions of years, these siliceous shells have accumulated at the bottom of their aquatic habitats, forming vast sedimentary deposits known as diatomaceous earth or diatomite.