I didn't see a question concerning diatoms, but folks seem  to be responding
to one??  So here is a little information.  Diatoms are microscopic
single-celled algae, which have a cell wall composed of silica.  They are an
important food source for many browsing types of snails, both in fresh water
and marine habitats.  Snails which live on seaweeds for example, like
Littorina and Lacuna, as well as snails which live on lily pads and other
fresh water plants, do not usually eat the seaweed or plant itself, but
rather browse on the diatoms and other unicellular algae which live in the
slimy secretions on the surface of the larger organism.  The cell wall, or
"shell" of a diatom is made in two halves which fit together like the top
and bottom of a friction-fit plastic box, one half overlapping the other.
The silica walls are inscribed with extremely fine striations, the spacing
of which is extremely constant in a given species.  The ability to visually
distinguish these striations is used as a measure of the resolving power of
various microscopes and other optical instruments. In some parts of the
oceans, the sediment is composed largely of empty diatom shells.  The silica
does not biodegrade, and is chemically inert, so they tend to remain
virtually forever, and are common as fossils.
Ordinarily, organisms the size of diatoms, which can be readily seen with a
standard student microscope, would not be measured in angstroms, but in
microns (=micrometers).  A micron is one thousandth of a millimeter (or a
millionth of a meter).  An angstrom is one ten-thousandth of a micron, or or
one ten-millionth of a millimeter, a distance which is too small to be
useful in light microscopy, but which is used in electron microscopy, as
well as for measuring wavelengths of electromagnetic radiation.
I hope this relates to the original question, which must have slipped by me
somehow.
To put this in perspective, the world's smallest snail shell, Ammonicera
rota, measures a whopping 5 MILLION angstroms, while the world's largest
gastropod, Syrinx aruanus, can reach a length of 7,500,000,000 angstroms!

Regards,
Paul M.