Moulded Glass Rock Slides are individually hand moulded in the USA by master lampworkers.  These have all the same features as the brass Rock Slides® with the Knuckle Cut, Finger Rest and Tapered Interior.  These fit almost exactly like their brass counterpart but are slightly longer.  The main difference being that they are much lighter & faster so you can really cook with these on an electric guitar. Because of the unique tapered interior design they do have enough mass to light up your acoustic.