Piece of Thermal Insulated Borosilicate Glass Cup. The product features double layer glass and thermosil. Heat-resistant, Stylish, Elegant and Double-Walled Design. It is made of high quality borosilicate glass. Don't let your coffee get cold at home, at work or in the office. 250 ml Borosilicate Glass has a granite-like structure, is very difficult to wear, does not scratch It is resistant to high temperatures as well as low temperatures. There is a round mark on the bottom of the glass due to the construction of the double-walled glass. It's not a defect!!?