A drop of tin, a speck of gold. The tiniest trace of any so-called conflict mineral in a vast array of products — everything from buttons to cellphones to jet turbines — is now under the microscope of the U.S. Securities and Exchange Commission.

This month, new SEC rules went into effect that will require about 6,000 publicly traded companies to report whether their products contain four conflict minerals from the Demo­cratic Republic of the Congo, where ­warlords are using proceeds from the mineral sales to fund a horrific campaign of murder, rape and torture.