Green’s Dictionary of Slang

glassy (alley), the n.

also agate, glassy agate, glassey, glassy marble, glassy peb, juicy onion
[marbles use glass alley, a specially prized type of marble + SE glassy, of a surface, smooth, unruffled, absolutely flat]

[late 19C+] (Aus.) the best, the favourite, the most admired; usu. in phr. just the glassy marble, etc, exactgly as required, wholly satisfactory.