Fascinating project!!! First of all, glass does not have crystal planes, so any time you hit it, it will break in an irregular pattern that sort of looks like circular fractures. We call this "conchoidal fracture".

A true crystalline solid, like calcite, will always break along its crystal planes, no matter where or how, or how hard you hit it. You might say that glass breaks "chaotically". That means that a very minute change in the way you hit the glass will result in an unpredictable way in which it breaks.

You can try to set up the breaking conditions very carefully: you can take a polished pointed weight, drop it from a precisely measured height in precisely the same way every time, but there are tiny variations in exactly the angle at which it hits the glass that you cannot control. Because glass is not a crystalline solid, the way it breaks will be sensitive to the very minute variations in the breaking conditions. And so it will never break the same way twice. Perhaps you have coined a new saying: "Glass never breaks the same way twice." Congratulations!

There are two reasons I can see that your glass did not break the same way. One is that even though you tried your best to cut the squares exactly and drop the weight exactly the same they are probably still slightly different. The other reason is in the actual formation of the glass. When glass is made the particles will arrange in a slightly different manner making each piece stronger or weaker in different spots. So, the weaker places are more likely to fracture, each time creating a different pattern.

This is a very interesting experiment. Glass breaks (fractures) in what is called a conchoidal manner and the reason for this is related to the fact that glass has a very special kind of disorder in the structure. One of the characteristics of conchoidal fracture is that it is difficult to predict how exactly it will take place. Researchers are still working on precisely the same experiment you have described; so many congratulations for having picked a fascinating problem to work on.

Your question is a good geology question. Glass is amorphous, meaning that it's atoms are not arranged in an orderly manner. Minerals, on the other hand, form in an organized way and their atoms are lined up in flat sheets or in cubes, for example. This pattern repeats itself over and over as the crystal grows.

Glass is sort of like a mineral in that it is hard and can occur naturally (like volcanic glass) but glass is not crystalline (doesn't grow in crystals made up of atoms with a specific arrangement).

In a mineral crystal, some atoms are bonded more strongly than others. Where the bonds are weak, the crystal is most likely to fracture if you smash it. Take halite (salt), for example. It's weakest planes are all 90 degrees from each other so that the mineral breaks into perfect cubes. The cubes may be different sizes but they always have the same shape.

You can test me. Use a magnifying glass and take a look at some salt from your salt shaker at home. You'll see that all or most of the salt pieces are little, tiny cubes. (They may not be perfect cubes anymore if they rubbed on each other too much, but they'll be close). Because glass is not crystalline, it breaks into many different shaped pieces. I hope this helps!

This is because glass has a property called conchoidal fracturing. That means that it doesn't break in a straight line, but in almost a spiral pattern. If you break a square of say halite (salt crystals) they always break into smaller squares. This is due the arrangement of atoms within. Glass has an arrangement of atoms that is equally strong in all directions so it won't break on the same line each time.