|Googled 'Talisman'. There you go|
Talisman of Arthas Moloch
Quite an interesting piece of wargear, especially to a faction that has no other anti-psyker capabilities. When your faction has generally low stats, you are particularly vulnerable to enemy psychic attacks. This wargear helps out more than you'd think; and has saved my Riptides more than once. It is also quite handy as a poor-man's shield generator when you don't have the room in your support systems.
Since I often take this piece of kit to complement my Fusion Blades, I decided it deserved representation on the model as well.
Besides, this doesn't actually have to be THE Talisman. I'm sure there are lots of protective talismans like it in the universe.
Since Tau are always tinkering, I imagined them trying to 'harness' the protective energies and augmenting them with tech they already have: shield generators. In this way, I could show why it has a 5+ invulnerable save as well. So I decided I would embed the stone in a custom shield generator, similar to Farsight's but smaller.
So perhaps this particular talisman was found by my Escort Cadre, in a temple. However it doesn't provide the physical protection the original provides, and is a weaker talisman overall. So it is embedded in a stripped down Tau shield and amplified to produce the same effect. Tadaa! Fluff win.
Converting the Talisman
I glued the stone in, added a magnet to the back, and I was finished! Not all that difficult really.