White nylon plastic with a matte finish and slight grainy feel.
About this Product
This is a custom brick to allow an Arduino UNO to connect to Lego* & compatible bricks. I have only tried this part in the White, Strong Flexible material. It connects securely to both the Arduino UNO and to other Lego bricks while still being removable without undue effort. Other materials may work better or worse, if you try a different material, please let me know how it works out.