Glass blocks are small (usually under 12 inch x 12 inch)
bricks of
glass that are usually combined together in a
grid as part of a
wall (like a window). They have the advantage that they allow
light to filter through, but maintain
privacy, since you can't clearly look through them. They have a high
insulation rating (similiar to a 12 inch-thick
concrete wall), are more
secure than normal windows, and are good at
absorbing noise.
Some co-workers have informed me that glass blocks are now illegal in some areas since they are (incorrectly) considered to be the same as normal windows, but since they can't be "opened", they do not provide an escape route in the event of a fire emergency.