Volcanic glass shard fragment. Imagine glass whipped into a foam composed mainly of thin glass bubbles. Break that up. What remains are thin curved pieces of glass and the triple junction pieces where several bubbles came together. The fragments may display concoidial fractures (those scallop shaped fractures in glass).

Vesicle wall fragment. Just the inner wall of a bubble.

A portion of intersticial cement cast of unknown crystal/clast. If you have a pile of spheres (or other shapes), there is room between them for other stuff. This is where cementing minerals are precipitated. Break off those spheres and you are left with the cast of the removed object on the cement. The missing object wasn't a sphere, but a more complex shape.

Remember scale: this thing is really, really small.