Normally appears to be a simple, smoky grey crystal, but displays a quality known as the schiller effect - a metallic luster - when viewed from certain angles. These metallic glints are violet, blue, and green; but sometimes yellow, orange and red can be seen.
This alone would make it valuable as an ornamental stone, but the trace amounts of eezo in its structure make it even more so.
Aside from obvious potential as a semi-precious stone, Ulessian Feldspar is also commonly used to create meditation aids for biotics.
For such a purpose, the stone is usually shaped into a cabochon sized to fit easily into a adult asari's palm, as a child typically recieves only one to use over a lifetime.
In practice, a biotic holds the crystal at about eye-level, and attempts to spin it rapidly without fluctuation in speed or height. If done properly the stone should spin like a top, and the traces of eezo should flash and spark in reaction to the biotic charge.This requires a lot more skill than the description suggests, and is considered a good early step in biotics training as well as a way to calm the mind.
There are some similarities in the terran Singing Bowl meditation, and in fact singing bowls have been adapted for similar use.