Although there hasn’t been any details about it specifically in D2, I can only assume it may be the powdered remains of Bright Engrams, infused with an indiscernible amount of Light.
If it’s a currency in game then it definitely would be of some sort of researchable value to the Cryptarchs or perhaps to the Awoken in some regard.
It was a long time back, but I do remember there being a small amount of lore regarding Tess and her uncle.
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Oh, this sounds cool and fit pretty well as possible description. Thanks!