Assertion: opal | /r/HasProperty | stone
Facets:
value: in the middle ages
type: TEMPORAL
Source sentences:
text: Opal is a stone that will require care that does not fragment its surface.
text: In the Middle Ages, opal was considered a stone that could provide great luck because it was believed to possess all the virtues of each gemstone whose color was represented in the color spectrum of the opal.