The Three Prime Essentials are Salt, Sulfur and Mercury (Quicksilver).
At the end of The Great Work, the Three Primes are reunited and integrated into the Philosopher’s Stone.
“The Stone is single in essence, but triple in form.”
Sulfur and Mercury are the primordial opposites that are part of the inherent duality of the universe. You only have to observe nature to see this fundamental duality, which can be expressed as masculine and feminine, positive and negative, light and dark, expanding and contracting, energy and matter, Yang and Yin.