Elka Watches are equipped with Chevé box sapphire crystal.
The design of this shape is relatively complicated, namely the hardness of sapphire is 2000 HV, which makes it probably the hardest material just after diamond.
The choice of this type of shape was a must in order to take up the codes of the 60's, a period during which watch crystals were made of Hezalite/Plexiglass, thus easily formed in a mould. The glasses had elegant shapes, they were curved. The advantage was the malleability of this non-hard material which allowed to do many things. Today we are used to scratch resistant watches hence the need of using sapphire.
However, the process is more complicated, requiring diamond tools to be able to machine the material and "sculpt" it to give this visual aspect of yesteryear.
The big advantage of this shape is a generous opening on the dial and a thin aspect of the watch.