Adam Clarke on Deuteronomy 25:13

Divers weights - אבן ואבן eben vaaben, a stone and a stone, because the weights were anciently made of stone, and some had two sets of stones, a light and a heavy. With the latter they bought their wares, by the former they sold them. In our own country this was once a common case; smooth, round, or oval stones were generally chosen by the simple country people for selling their wares, especially such as were sold in pounds and half pounds. And hence the term a stone weight, which is still in use, though lead or iron be the matter that is used as a counterpoise: but the name itself shows us that a stone of a certain weight was the material formerly used as a weight. See the notes on Lev 19:35, Lev 19:36.