|Aventurine is actually not a cryptocrystalline quartz like an agate or jasper. It is a metamorphic rock (a form of sandstone that has experienced a metamorphic process) quartzite. What makes it unique is that aventurine incorporates a multitude of small impurities that might include platelets of mica, hematite, or other sparkely inclusions. The green variety likely contains mica or fuchsite (a chromium containing mica). The chromium aiding in the green coloration. The platelets reflect light through the translucent quartz creating a sparkling effect.
Below are two micro-images of the platelets in this sample.