If the edges of a glass block were rounded instead of square what would happen to the density?
Nothing. Density of a particular substance remains the same no matter how much of the substance is present or what shape it is in. Since density is measured in mass / volume, adding or taking away material adds both mass and volume or subtracts both mass and volume, keeping a constant proportion.
Density = Mass / Volume To increase density, you could increase mass or reduce volume. To increase mass, you could use a different material (metal instead of plastic) to make the object. To decrease the volume, you could change its shape (square to round) or compress it (force all the air out if there is).