Blue Aventurine is a variety of quartz. Coarsely crystalline quartz is common in many rocks. Because of crystalline quartz's mechanical and chemical stability, it endures as detrital grains when quartz-bearing rocks break down. These grains can accumulate as sand and pebbles in sedimentary deposits. Some sedimentary deposits are composed almost entirely of quartz grains which can coagulate over time, leading to the formation of sandstones. When a quartz-rich sedimentary deposit or a sandstone undergoes high-grade metamorphism (metamorphism is the alteration of a rock due to changes in heat, pressure, or chemical environment), the original material alters to a compact rock composed of interlocking quartz grains known as quartzite. Aventurine gets its spangled crystal appearance from the sparkling internal reflections from uniformly oriented, tiny inclusions of other minerals. Depending on what impurities are present in the original sandstone or sedimentary deposit, if aluminous sediments that contain Boron are present, then Dumortierite can occur and disseminate (scatter) within the Quartzite, giving the Aventurine its blue color.
Sources:
“Blue Aventurine Quartz: Mineral Information Page.” Blue Aventurine Quartz Factsheet and Information Page, www.mineralminers.com/html/avbminfo.htm#blue-aventurine-quartz-physical-properties.
