Most retailers will insure your item until you sign for it upon arrival. I didn''t bother insuring it with Chubb until I verified I got what I purchased (independent appraiser).
I had my ring insured before I even got it, but it depends on the situation. If you''re planning to take your ring to an independent appraiser and it is a possibility that you will return it, then I would consider holding off for now. If, on the other hand, this is THE ring, then by all means insure it now.
Chubb has the best payout policy, in popular opinion. Jeweler''s Mutual also gets good reviews around here. You can access Chubb through Touchstone. Keep in mind that stand-alone jewelry policies with Chubb must have a minimum value of $10k, and you will pay a bit more for the better claims settlement provision.