Terrigen Crystals are an incredibly rare mineral that gives rise to the Terrigen Mist. Guarded by the Inhumans. These crystals form naturally in the subterranean Terrigen Waters on Earth's moon near the city of Attilan. At the correct temperature, when exposed to water the crystals react to produce the Terrigen Mist vapor. These mists are pumped into flux chambers during the process of " terrigenesis." The crystals themselves are kept in a sealed cavern below the surface of Attilan, where they are constantly exposed to heat and saline water.
Discovered 25,000 years ago by the Inhuman Randac, he exposed himself to the mists created by the crystals and received incredible mental powers. Those who undergo the genetic mutation of Terrigenesis are monitored carefully by the Genetic Council. Each individual's mutations vary. All of the Inhumans were exposed to the mist, granting them their respective powers.
In Other Media
At the start of the second season of the show, a flashback to World War 2 shows Peggy Carter and the Howling Commandos recovering a mysterious Kree Obelisk from a captured HYDRA facility. In the present, the item is stolen by the agents of S.H.I.E.L.D., who seek to keep it out of HYDRA's hands. The Obelisk is shown to be fatal to any human who touches it, which leads to Isabelle Hartley suffering a severe injury after grabbing it. During the ensuing chase, the artifact is stolen by the Absorbing Man, who kills Hartley and Agent Idaho.
In the subsequent episodes, HYDRA weaponizes the Obelisk and uses it to create small disks which can instantly kill any human upon contact, which are used by Marcus Scarlotti and his crew. Calvin Zabo later informs Daniel Whitehall that the Obelisk is actually called the Diviner, and that it was left behind by the Kree Empire in order to determine which humans were genetically "worthy".
In the mid-season finale, the Diviner is activated and opens up to reveal a Terrigen Crystal, which activates Terrigenisis in Skye (whose real name is revealed to be Daisy Johnson) and Raina, revealing both women to be Inhumans.