Assertion: silica | /r/AtLocation | five crystalline forms
Facets:
value: opal in an amorphous form
type: OTHER
value: naturally
type: MANNER
Source sentences:
text: Silica occurs naturally in five crystalline forms (quartz, tridymite, cristobalite, coesite, and stishovite), in a cryptocrystalline form (chalcedony), and in an amorphous form (opal).