The Eldar spirit stones are called Waystones. A spirit stone is a small, seemingly precious stone carried on an Eldar's chest, used to save their souls from destruction by Slaanesh. At the moment of the Eldar's death the stone acts like a 'psychic trap' absorbing his psychic self and preventing it from entering into the Warp. This prevents an Eldar's soul from being consumed by the nemesis of the Eldar race: Slaanesh.
The Waystone can then be brought back to the Eldar's own Craftworld and embedded into its wraithbone core where it will grow into a larger spirit stone. Once the stone is implanted, the Eldar's soul can travel freely through the wraithbone, mingling with other Eldar souls and forming part of the communal spirit of the Craftworld itself. All the souls within a Craftworld are collectively known as the Infinity Circuit.
In times of dire need, the Eldar are also able to retrieve the spirit stones of long dead Eldar heroes and place them into the artificial bodies of Wraithguards and Wraithlords. These constructs are then piloted by the Eldar soul, which experiences reality as a dream. This process is considered necromancy in Eldar culture and only done as a last resort. The Eldar Warlocks who specialize in this process are known as Spiritseers. Iyanden, an Eldar Craftworld with a very small population is known for its greater use of these 'living dead'.
It is believed that when enough Eldar souls have been gathered within the Infinity Circuits of the craftworlds, a new god will emerge; Ynnead Eldar God of the Dead. It is hoped by Farseers that this god will be powerful enough to vanquish Slaanesh, freeing the Eldar from their doom.
Spirit Stones are also an Eldar vehicle upgrade which allows the spirit to control the vehicle in case the pilots are incapacitated, in a similar way to the Power of the Machine Spirit.
There is one primary source of spirit stones: the old Eldar homeworlds now known as Crone Worlds. Acquiring new spirit stones is extremely dangerous because the Crone Worlds exist within the Eye of Terror. In Eldar mythology the spirit stones were created by Vaul from the Tears of Isha. This is taken as a metaphor for the crystallization of psychic energies of the Eye.
Notes
The graphic novel Daemonifuge features these stones passingly, but names them Soul-Stones. However the Eldar Codices name them as Spirit Stones4.