Assertion: black opal | /r/HasProperty | worthless
Facets:
value: completely
type: DEGREE
Source sentences:
text: Black opals are the most precious type of opals, they have body color of black or dark gray (do note that black opal should not be completely black otherwise it would be completely worthless, it should displays a myriad of rainbow colors, the more colorful the better), sometimes dark green or blue are also called black opals.