Assertion: tourmaline's color | /r/HasProperty | improved by heat treatment
Facets:
value: sometimes
type: TEMPORAL
Source sentences:
text: The color of tourmaline can sometimes be improved by heat treatment and or irradiation.
text: The strong dichroism of most green tourmaline is such that the optic-axis direction is almost opaque; by heat treatment, however, the color of some can be improved and the absorption reduced.