Can an Eldar's waystone be lost without being retrieved or destroyed ? If it does, is it like an inert material (with a spirit potential inside) ?
The Eldar do not make a point of losing spirit stones or waystones. It may happen but only under the most dire circumstances. Since there is no source of new waystone material, no Eldar wants to be responsible for losing this most precious resource. They are also reputed to be potent sources of psychic energy.
There is one primary source of Spirit Stones: the old Eldar homeworlds that lie within the boundaries of the Eye of Terror and are now known as the 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 the Eldar God Vaul from the Tears of Isha. This tale is taken as a metaphor by the Eldar for the crystallisation of the psychic energies of the Eye of Terror into the Spirit Stones. --Warhammer Wikia > Spirit Stone
A waystone that is found houses the soul of a long-lived companion who may have knowledge and experience precious to the Eldar people. The Eldar take the idea of not leaving a man behind to the extreme. If a body cannot be recovered, a waystone WILL be. Even in pitched battle, they tend to keep paths of retreat open and have cloaked agents who will make every effort to gather waystones from a battlefield.
With that said, a battle that goes horribly wrong or if the Eldar are caught without retreat, or in the case of certain kinds of weapons, their waystone is destroyed on the battlefield, that Eldar dies and risks being consumed by Slannesh. It is not guaranteed, but it is a possibility.
Eldar waystones can be lost on battlefields, in spacecraft, or on abandoned craftworlds and if found by other Eldar, and the two factions are not incompatible, they may be reintroduced to THEIR Infinity Circuit to begin the cycle of rebirth or reuse as a Wraithlord or other automaton.
If they are incompatible, those waystones may be given a place of honor, but not recycled. Only the Dark Eldar would destroy the waystones of a fallen Eldar not their own.
Yes, spiritstones can be lost/destroyed. When this happens, the eldar they contained is pretty much dead. The spiritstones act as a buffer between the eldar and Slaanesh. It grabs the eldar's soul upon death, and holds it in safekeeping until it can be returned to the craftworld.
HOWEVER, if a spiritstone is destroyed while occupied, that buffer is gone, and the eldar's spirit is lost into the warp and the waiting arms of Slaanesh. See the Blood Ravens Omnibus. When the ravens destory the Dark Reapers' ship, thousands of reaper souls were immediately consumed by Slaanesh (as they had all been stored on that vessel, not the craftworld).