Khums becomes obligatory (wājib) on five things:
1. profit from earnings;
2. mined products;
3. treasure troves;
4. lawful (ḥalāl) property that has become mixed with unlawful (ḥarām) property;
5. precious stones that are acquired by underwater diving.
Profit from earning includes:
• Salaries and wages
• Earnings from businesses
• Earnings from investments
• Welfare and sadaqah (alms given to the poor)
• Prizes and bonuses
• Unexpected legacy (something bequeathed to you in a will of someone not related to you)
But there are exceptions. Khums is not payable on the following:
• Mahr (the dowry given by a husband to his wife)
• Inheritance, unless the heir is certain that the deceased was committed to paying khums but just did not pay it from the estate before his death.