Incorrect. Diatoms grow their own frustules, and the shapes of those vary greatly between species.
To again post something from wikipedia because they phrase things so much better than I do:
When diatoms die and their organic material decomposes, the frustules sink to the bottom of the aquatic environment. This remnant material is diatomite or "Diatomaceous earth", and is used commercially as filters, mineral fillers, mechanical insecticide, in insulation material, anti-caking agents, as a fine abrasive, and other uses.
By the way, if you didn't catch it earlier, Frustules are "A unique feature of diatom cells is that they are enclosed within a cell wall made of silica (hydrated silicon dioxide)".