for the Messenger of Allah (peace and blessings be upon him) specified certain items and did not mention others. Thus, if we use the hadith as evidence for every item measured by volume or weight, we have used a more specific proof to establish a broader ruling.

Some scholars said that the effective cause in the four categories is edibility, and in gold and silver it is their function as a medium of exchange. This is the well-known position in the Shafi‘i school. Based on this, if one orange is exchanged for two oranges, it is not permissible, because oranges are edible. Likewise, if one ton of iron is exchanged for two tons of iron, this is permissible; whereas according to the first view, it is not permissible.

The closest view is that the effective cause with regard to gold and silver is simply that they are gold and silver, whether they are coins or not. The evidence that ribā applies to gold and silver even when they are not coins is the hadith concerning the necklace, narrated by Fuḍālah ibn ʿUbayd (may Allah be pleased with him): “He bought a necklace containing gold and beads for twelve dinars. He then separated it and found that it contained more, so the Prophet (peace and blessings be upon him) forbade its sale until it was separated.” (1)

It is well known that a necklace no longer serves as currency. Accordingly, ribā applies to gold and silver in all cases, whether they are coins, bullion, or jewelry, whatever form they may take. Ribā does not apply to iron, lead, brass, diamonds, or other kinds of minerals.

As for the effective cause in the four categories, it is that they are edible items measured by volume. That is, the effective cause is composed of two elements: measurement by volume and edibility, for this is their actual characteristic—they are edible items measured by volume. The effect of this disagreement becomes apparent in the examples:

If one sells one sāʿ of ishnān for two sāʿs of it, then if we say that the effective cause is