has anyone done "glass" filings/patches?

My ds (5) is having a tiny cavity filled or covered (not sure which) : with some sort of glass "filling" -might be wrong about it being a filling as no mention of drilling when dentist explained.
My ds is nervous and i assured him it wouldnt hurt since dr. told me that and i believed him. should I have ?
anyone know what im talking about and how did it go for you???
Laura