Mica

Mica is sometimes used as the dielectric in high-precision capacitors.

The mica particles in the stone glistened in the sunlight.

Oooh, look how the river sand sparkles! That's not gold -- it's little flecks of mica in the sand.

His little eyes glittered like mica discs with curiosity, though he tried to keep up a bit of superciliousness.