Materials are Considered Uncountable Nouns

Materials (glass, wood, iron, stone, paper, cloth, wine, coffee, tea &c.) are considered uncountable. But many of these nouns can also denote one particular thing, and then take an article:

Windows are made of glass but Have a glass of wine.
We write on paper but I've got a (news)paper.
Iron is a metal but I use an electric iron.

Some, any, a piece of, a lot of &c., are often used.

• Would you like some coffee?
• I want a piece of wood.