insuring watches, not sure about companies, as mine's through my house insurance, new for old policy.
But just in case the worst was to happen, I got Goldsmith's to write out a signed & dated 'new replacement cost' certificate (called something like that can't recall actual title). Which basically says, yes we've seen it, yes it is real yes he does own it & YES that's how much a new one is.
Which I get redone annually to keep everything up to date.