Aventurines are dark green gems found in sedimentary layers. They are not worth a great deal, only having a value multiplier of x3.
In real life
Strictly speaking a wider selection of quartzites and feldspars given a metallic sheen by mineral inclusions, Dwarf Fortress aventurines depict the typical aventurine quartz colored by fuchsite, a variety of muscovite. It is most often found in India and Norway.
[SOLID_DENSITY:2650] Common to quartz gems