Assertion: feather | /r/MadeOf | keratin
Facets:
value: mostly
type: DEGREE
Source sentences:
text: Feathers are made of keratin, the same material out of which a reptile’s scales and claws are made.
text: Feathers are mostly made of keratin, the protein that's responsible for the strength of wool, hair, fingernails, and hooves, he added.
text: Feathers are made of keratin, a protein which is also used to make horn and hair in different animals – and beaks in birds.