This diamond "guarantee" that you are referring to is NOT an admission of belief in responsibility for all lost stones, nor is it a statement reflecting a jeweler's belief in the invincible quality of a ring's craftsmanship--on the contrary, it is an insurance policy. YOU ARE THE ONE RESPONSIBLE for having the ring checked for wear and tear--otherwise--you are right--it voids your contract. (There are sometimes other stipulations to these policies that make the customer responsible, such as getting your ring stuck in a meat grinder . . .

) I'm familiar with these programs--they are known in the industry as a form of "jewelry insurance" (even though they come with the slick, customer-appealing title of "guarantee" or "warranty"), and the policies are offered to the jewelers to pass on to their customers through the same institutions that offer jewelers insurance for theft, fraud, etc. for their stores. Some jewelers offer the guarantees for a price to the customer (like the extended guarantees for refrigerators, cars, tvs, etc.), or the jeweler may present it as a "free service"--especially when making a large sale--and the cost is figured into the initial price of the stone. (A little tip from the industry--nothing--especially in the jewelry industry--is ever "free".

High-end retailers do not become high-end by giving the customer something for nothing.) Not that the outright or even the "hidden" cost of the guarantee isn't a better deal than getting a rider through your home insurance policy--it may be, depending on what kind of insurance you have, what your deductible is, etc. You'll have to investigate that for your own situation . . . But however you choose to purchase insurance, either through the jeweler or through a rider on your home insurance, I agree that it is always a good idea to get covered by insurance for any amount of money ($400 or $10,000) that you can not afford to take a risk on losing.
Oh, another helpful tip from the industry if you do decide to get another diamond--make sure that your stone/ring has been appraised by a Graduate Gemologist (a GG)--most high end jewelry stores have at least one--with credentials from a recognized institution such as the American Gemological Society (AGS), or that the stone has certificates from these recognized institutions. Otherwise, your stone has not been appraised by an "expert", and the appraisal may not be accepted by the insurance company. Certified GGs have proven their ability to grade and appraise stones and jewelry within a rigid set of industry guidelines. Terms such as master jeweler are generic and do not reflect any set amount of education or experience. Some mall jewelers use the term master jeweler to refer to employees who have been trained to use a diamond tester, memorize the 4 c's chart, and install watch batteries. Even the jewelry home parties use the term to refer to their consultants.
With all the "words to the wise" that you have been given by everyone (and I apologize if I got a little carried away with sharing things that I have learned from the inside), I really hope that your ring turns up, or that you are able to find another piece of jewelry that makes you happy and is a symbol of the beauty of your love!!!