Assertion: tourmaline's color | /r/HasProperty | permanent
Facets:
Source sentences:
text: Although the new color appears to be permanent, the stones develop cracks as they change color, and no method has been found that will prevent such damage, Green tourmaline showed no change as a result of bombardment.
text: Although some of these colors are improved by heating, the color is permanent and they are desirable as gemstones.