Diamond point engraving was a technique developed by the Dutch back in the 1600's, before mechanized engraving.
It is performed by scratching the glass with a diamond, each scratch creates a white line, allowing the creation of an image. 
It is very difficult on the hands to scribe the glass, due to the great pressure needed.

The other technique is traditional glass engraving, with water cooled stones which spin on a lathe like tool. 
The crystal vessel being engraved is held in the hands, and moved against the spinning stone, which cuts into the glass.