Here's why:

* Aventurine is formed from the alteration of other rocks. It typically starts as a metamorphic rock like schist or a igneous rock like basalt.

* The characteristic shimmer of aventurine is due to inclusions. These inclusions are tiny platelets of mica, hematite, or goethite that are embedded within the rock. These inclusions reflect light, creating the sparkling effect.

* Metamorphic rocks are formed from existing rocks that undergo changes due to heat, pressure, or chemical reactions. Aventurine's formation fits this description.

While it's possible for aventurine to have an igneous origin, the vast majority of aventurine found is metamorphic.