The surface of this kind of tube is smooth and the tube is colorless. The properties of the tube are: high purity( SiO2> 99.99% ) , abrasion resistance( any acid except HF acid ) . fire resistance( softening point 1700 centigrade) . low coefficient of thermal expansion( 5.5x10-7/ centigrade, heat the tube to around 1100 centigrade, then put it into ordinary temperature water, the tube won' t crack, ) ; The surface can be done with gold plating. Gently using sizes: single outer diameter X double outer diameter 8x17mm, 11x23mm, 15x33mm length 400-4000mm