From the Super Mario Wiki
Glass Blocks are rare objects that can be found in the Wario Land series, or more precisely in Wario Land II. They appear like small translucent blocks, apparently made of glass, and are very fragile. Glass Blocks can be picked up and used as a throwing weapon to defeat or stun enemies or to break Enemy Blocks. If the Glass Block falls to the ground or is attacked with a jump, it shatters. A new Glass Block can be found on its original resting place, though.
Wario Land series
Wario Land II
Glass Blocks appear in Wario Land II in a level called Defeat Four Ducks!. It is deployed in various rooms throughout Syrup Castle and can be used to progress through the fortress. The player needs to use the Glass Blocks to knock down one of the four D.D.s from an unreachable location, and to smash through a staple of Enemy Blocks.