Sulfur

Sulfur is an uncommon material added to the game. It is known for its yellow color and it can be found in a lot of foods, specifically egg yolks (no, you can't get sulfur this way in-game). Its formations are usually near lava and it is quite abundant in the Nether. Other than those locations, sulfur is rather uncommon, it can be purified out of crude oil in large quantities, but that requires the Oil Refinery.

Uses
Primarily used in lead-acid batteries (advanced batteries or the standard energy storage block) and such. Its block form can be used in Advanced Factory energy clusters too.

It is also used in Yellowcake, a critical step in Uranium processing and enrichment.