By itself, glass is in fact always breakable. Glass, by its definition is made out of materials that can at least crack. The only exception to this is when a hybrid material is made such as plexi-glass. This type of material can in fact be unbreakable, but it is not considered glass.
All glass is breakable, but there are special glasses.
There are glasses with iron in it to make it more secure when it brakes. Or there are "Panzerglass" translatet von German into English bullet-proof glass used in banks. This glass can also break but not that fast as the glass you use in your window.
There is news that Alfred University has signed an agreement with Santanoni Glass to produce unbreakable glassware that has produced the highest shatter resistant glass to date, however no glass is unbreakable, much like how nothing is really inflammable or flame retardant. The closest thing you can find to unbreakable glass is a car windshield, the outside breaks, inside intact.