crystal-like, [but] of silver - the measure whereof they alone will determine.

Crystal-clear, made of silver: they will determine the measure thereof (according to their wishes).

(Bright as) glass but (made) of silver, which they (themselves) have measured to the measure (of their deeds).

Crystal-clear, made of silver -- they have measured them according to a measure.

Clear glasses [made] from silver of which they have determined the measure.