After testing soil samples from gas stations with a Gram stain kit, we
examined it under a microscope without an oil emersion lens.  The
results we had gotten showed silver substances in the soil.  Our
question is, what is the silvery substance in the soil we found?

You really need to clarify. It is likely that the 'silver' substances
in the soil you see are simply mica grains that reflect light well.
You don't say what type of microscope you are using (they can be
reflective or refractive) or what magnification, so there is little
clue as to the scale of these substances. Mica is common in soil in
some areas and can be sand size, an example:
http://crops.confex.com/crops/2005am/techprogram/P5281.HTM Knowing
your general geographic area would help to explain the soil contents:
e.g.: http://www.soil.ncsu.edu/about/century/1979.html  Information
about mica minerals here:
http://www.soil.ncsu.edu/about/century/1979.html