When it comes to tube and rod materials, transparent or not, sapphire tubes and sapphire rods are the go to material in so many applications. 
Exceptional applies to sapphire's performance under numerous conditions: mechanical, optical, thermal and chemical. 
In these cases, a sapphire tube or sapphire rod is without a peer. 
Sapphire rods are superior for rod bearing and standoff applications that involve high abrasion and wear while maintaining dimensional stability.

Why choose the sapphire?
1-Thinner and Stronger than Standard Glass Windows
2-Transmits Wavelengths Ranging From UV to Mid-Infrared
3-Features Extreme Surface Hardness and Chemical Resistance

Sapphire Parts are manufactured from single crystal sapphire, making them ideal for demanding applications (such as laser systems) because of their extreme surface hardness, high thermal conductivity, high dielectric constant and resistance to common chemical acids and alkalis. Sapphire is the second hardest crystal next to diamonds and, because of their structural strength, sapphire windows can be made much thinner than other common dielectric windows with improved transmittance. Chemically, sapphire is single crystal aluminum oxide (Al2O3), and is useful in a transmission range from 0.2 - 5.5µm.