Flashback 3: Starfleet Academy
Stratospheric micrometeorites have diverse structures, solar flare tracks, and pores for cometary ice.
From most to least common in the stratosphere are: aluminum oxide from solid fuel rockets, porous silicates, and meteoritic iron.
In meteorites we find ancient diamonds, silicon carbides, & graphite “onions” from other stars.
http://www.umsl.edu/~fraundor/aapt98x3.html
http://www.ciw.edu/lrn/psg.html