Gems obtained by sea diving
1: If pearls, corals or other gems are obtained from the sea-bed by diving, whether it is mineral or a growth, if it reaches 3/4 mithqal of gold in value (= 3.51 g.) Khums should be paid on it, regardless of whether it was brought up after a single dive or more.
But if the gems were brought up in two different diving seasons, and in each case, the minimum value limit of 3.51 g. of gold was not reached, it will not be obligatory to pay Khums on either. Similarly, when diving is done in partnership, and the share of each partner is not commensurate with 3.51 g. of gold in value, Khums will not be obligatory upon them.
2: If a person takes out gems from the sea mechanically without diving, it is obligatory on him, as a precaution, to pay Khums on it. But, it he obtains them from the surface of the sea or from the sea-shore, he should pay Khums if his income from this source alone, or in combination with other profits made by him, exceeds his expenses for one year.
These laws are according to Aayatullah Sistani's Fataawa.