It's a little risky to say anything too specific about "silicone", because
silicone is not a single substance.  It's a collective name for a wide array
of compounds, with quite different properties (for example - waterproofing
agents,  various lubricants, vacuum grease, hydraulic fluid, defoaming
agents, anti-gas medications, window and boat caulking, and the "silly
putty" kids play with are all silicone).  I assume you are using one of the
clear, low viscosity products akin to waterproofing compounds.  Most
silicone compounds do share certain properties which are favorable to use on
shells, namely chemical stability and inertness (they don't tend to break
down or react with other substances), and lack of pH (no acidity).  However,
obviously not all the above types of silicone are applicable to shells, so
using a particular silicone product will require a bit of
trial-and-hopefully-not-error, to determine its efficacy.  In general, I
can't think of any characteristics of silicone products that would be likely
to harm shells.  This is also true of some oils - but not others!  Oils of
plant or animal origin should especially be avoided.

Paul M.