First, explain that you don’t want to lessen the value of your diamonds or their diamond, so you can’t discount on diamonds.
Then offer the customer a “Coupon” for an acceptable amount of a discount off any other piece(s) of jewelry at any time in the next year.
For instance, instead of giving a $300 discount on a $3,000 diamond, give them a coupon worth $300 off of $600 worth of jewelry.
The $600 does not have to be purchased all at once, but all must be used within a year.
The store keeps their “no discount” policy in tact.
The store does not lose any money, maybe even makes some money on the “coupon” sales.
The customer is happy because they are “getting something extra”.
The store gets the customer to return to the store at least one more time, and maybe more…