In general I have found them to be reminiscent to (or a nod in the direction of) birthstones in our life/reality.
Each Eldar is born and is able to be attuned to a particular type of Spirit(Birth) stone, they may be surgically implanted - which I doubt since it seems that Slaanesh IS capable to capture Eldar before their souls are "transplanted" into the stones, however rare that happens - or more likely I think are a trinket/meditation tool/special item that each specific user utilizes in his own special way. Some may wear it as an ornament, others may embed it into their armor (Looking at you Aspect warriors) and others may simply leave it in their room.
----
Side tracked
Since Slaanesh feeds upon emotions, and that the Craftworld Eldar are...in short attempting to find inner tranquility I believe that should a member of the Eldar race die and feel anything that they regret, where they have found inner peace during the moments that they die THAT is when Slaanesh steals their souls, leaving behind an empty (possibly reusable?) soul stone. Most Eldar tend to be able to accept their death and are able to enter their soul stone; which explains/helps the credibility for the reason why they Eldar are terrified of Slaanesh - She is ALWAYS thirsty, and for all intensive purposes is same thing that wants to eat you and is giving you a blow job at the same time, waiting for you to crack...hopefully that makes sense...I watched Swordfish the other day, and THAT (You know the one, or at least should since I basically said it) scene is what is effectively happening in my mind.
On Slaanesh's Omnipotence...While I am one of those who believe that Slaanesh "knows" all things that interest him most of the fleeting moments where humanity unwittingly generates power for Slaanesh is not a sustaining amount, and since the person is not willing to debase himself/others Slaanesh cares not. A person having Erotic asphyxiation and is willing to die to enjoy the most pleasurable experience is a person who Slaanesh is more invested in, or at least gains more power/energy from, than a person who is celibate, who for all intensive purposes is "invisible" to Slaanesh.
----
Back on topic
A part of my reasoning on why it's on the individuals temperance when they die, is I do believe (or at least think that I read it somewhere) that the Harlequins do NOT carry soul stones, and that they 99.9% (I can't recall if the person who plays the role of Slaanesh in their dance offers themselves when they die or not) of the time remain unable to be captured by Slaanesh; this might have to do with the fact that the Laughing god was not defeated by Slaanesh.
----
Anyways it's 5am here and I need to sleep.
Each Eldar is born and is able to be attuned to a particular type of Spirit(Birth) stone, they may be surgically implanted - which I doubt since it seems that Slaanesh IS capable to capture Eldar before their souls are "transplanted" into the stones, however rare that happens - or more likely I think are a trinket/meditation tool/special item that each specific user utilizes in his own special way. Some may wear it as an ornament, others may embed it into their armor (Looking at you Aspect warriors) and others may simply leave it in their room.
----
Side tracked
Since Slaanesh feeds upon emotions, and that the Craftworld Eldar are...in short attempting to find inner tranquility I believe that should a member of the Eldar race die and feel anything that they regret, where they have found inner peace during the moments that they die THAT is when Slaanesh steals their souls, leaving behind an empty (possibly reusable?) soul stone. Most Eldar tend to be able to accept their death and are able to enter their soul stone; which explains/helps the credibility for the reason why they Eldar are terrified of Slaanesh - She is ALWAYS thirsty, and for all intensive purposes is same thing that wants to eat you and is giving you a blow job at the same time, waiting for you to crack...hopefully that makes sense...I watched Swordfish the other day, and THAT (You know the one, or at least should since I basically said it) scene is what is effectively happening in my mind.
On Slaanesh's Omnipotence...While I am one of those who believe that Slaanesh "knows" all things that interest him most of the fleeting moments where humanity unwittingly generates power for Slaanesh is not a sustaining amount, and since the person is not willing to debase himself/others Slaanesh cares not. A person having Erotic asphyxiation and is willing to die to enjoy the most pleasurable experience is a person who Slaanesh is more invested in, or at least gains more power/energy from, than a person who is celibate, who for all intensive purposes is "invisible" to Slaanesh.
----
Back on topic
A part of my reasoning on why it's on the individuals temperance when they die, is I do believe (or at least think that I read it somewhere) that the Harlequins do NOT carry soul stones, and that they 99.9% (I can't recall if the person who plays the role of Slaanesh in their dance offers themselves when they die or not) of the time remain unable to be captured by Slaanesh; this might have to do with the fact that the Laughing god was not defeated by Slaanesh.
----
Anyways it's 5am here and I need to sleep.