This is two of my collection, I bought this in France in 1989. One in Thailand World Trade Centre.
The advantage of Turtle shell is its extreme lightness and its nonallgeric material properties.
Since the 80's most of the countries had prohibited the import of Turtle shell as to protect this species. So the turtle glasses are very rare. Some old Chinese believe the turtle shell has some kind of power to pretect the man using it.. that was one of the reason of its popularity and the cause it go to extinct much faster in the 70's.