Glass Ice cubes?

surely something like the strange blue plastic freezer blocks in minature form would be ideal for cooling drinks without watering it down?

Water has a very high heat capacity meaning that it takes alot of energy to raise one gram of it by one degree celcius. Glass, depending on the type, may have a very low heat capacity and thus would not be much help. It also cracks when exposed to extreme temperature ranges in a short amount of time. If you don't want your drink watered down put the ice into a thermal resistant plastic coating and designa machine to fill them up.

Glass cracks and shatters in sudden temperature changes. So they may well crack as soon as they were put in the drink.

They'd also smash easily and that could pose a health hazard.

They may scratch the inside of a glass.

Glass doesn't get as cold, and takes less time to warm up - ice stays colder for longer.

Surely glass wouldn't absorb and retain the cold, like water does...?

Why not use the little, liquid filled, plastic cubes, which you freeze. Happy medium, maybe?

Have you never seen those plastic ice cubes? They are plastic cubes filled with water.

Much cheaper and more robust than glass.