Though mass is closely related to weight, it is not specifically the weight of an object. Weight is a force comprised of both mass and the acceleration due to gravity. Scales that use springs to measure mass convert the force of weight into mass using a pre-determined adjustment. Scales that are counterbalances, on the other hand compare mass to mass directly because gravity on both sides of the scale match such that the effects of gravity can be ignored.
So weight is a force which naturally includes acceleration. Whereas mass is merely the measure of resistance to such acceleration (merely a measure of inertia).
As a specific example of how mass and weight are related but different, think of a plastic bag being filled with water from some sort of syringe. As the fluid is injected into the bag, the bag hangs heavy with the weight of water. But now imagine the same bag being filled aboard a space station out in orbit around the Earth. Now as the bag is filled, the bag merely inflates. There is no force of weight to make the bag hang heavily. But in both cases the mass is the same.
So, if someone wants to classify something by mass, it can be thought of as being somewhat similar to classifying the item by weight except that the pulling tension of weight isn’t involved. In the calculators I’ve provided, the gold is classified by mass rather than weight.