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.