Kay Jewelers should be accountable for the quality of both their products and their staff. If disreputable people are being hired and replacing customers' genuine diamonds with fakes, then Kay should be responsible. The same can be said if they are not doing thorough quality checks on their products. A company should be held accountable for what happens behind its doors.
Kay Jewelers should be held accountable for the actions of their store employees and anything that happened to the jewelry of Kay Jewelers customers when those customers left their jewelry in the care of one of the Kay Jewelers stores. If Kay Jewelers wants to be considered a reputable place to purchase jewelry, they should pay for the replaced diamonds and some restitution.
Kay Jewelers is responsible for the switching out of fake diamonds, because they are responsible for their employees. They should have had an audit system in place to catch people switching out the diamonds. They must have known that people can be dishonest, working with such valuable items such as diamonds.
Kay Jewelers should be responsible for the diamonds that were replaced with fakes after people entrusted their jewelry to Kay's for repair. This is the worst crime i have heard of and apart from being responsible for the diamonds, they ought to face the law. This is a high degree of fraud. It is stealing from the innocent citizens.