Aventurine is quartzite — a metamorphosed sandstone — with inclusions of fuchsite mica, and the sparkle comes from light reflecting off the flat faces of those mica flakes inside the stone. The same effect in glass is called goldstone, and the two are sometimes confused.
Worth being explicit: this is quartzite, not jade. Chinese trade names for this material include "Dongling jade," which is a commercial name rather than a mineral one, and English-language listings that call it jade are how mis-description claims start. Here it is called quartzite.