You should first check with the company who has your home/renter's insurance and auto insurance. The price all depends on the value of the ring and then of course the deductible you are willing to assume (the larger the deductible the lower the premium...usually $100, $250, $500 are common).
I work for State Farm and we usually recommend people insure their engagement rings on a seperate policy (personal articles policy) and with other companies it's called a rider/floater onto their home insurance. The reason is as someone else stated that this policy will cover many more things than just having it insured under the jewelry coverage on the home insurance. The home insurance will just cover usually up to 1,000 or 2,500 and for only fire, vandalism, theft whereas the pesonal articles policy will cover if it's lost or the stone chips or you run over it or pretty much anything which is better because this isn't a piece of jewelry you store in a jewelry box, it's something you where everyday and anything can happen to it!
I don't know about jewelers mutual but I'm sure they are fine and obviously specialty...but it doesn't hurt to check with your home and auto insurer because of the discounts that may be available.