gallnut
/ˈɡɔlˌnʌt/
noun
- A round growth on oak trees, caused by insects, used to make ink and dye.
- Historically, gallnuts were an important ingredient in iron gall ink used for writing manuscripts.
- The tree was covered in small, hard gallnuts after the wasps laid their eggs.
- The artist ground gallnuts to make a rich black ink for his drawings.