A bouncy ball is a lot harder to break with a hammer than glass marble.
that is because the elasticity of the bouncy ball increases the amount of time the force is absorbed, so the force is more gradual, whereas a glass marble is brittle, so it absorbs most of the force almost instantaneously.
from what i've always interpreted, from the way the shield wobbles(if you look closely), i've always thought it was some kinda dense fluids that absorbs incoming fire, as the shield absorbs the bullet/rocket/whatever, it creates a fluctuation wave, that is easily dissipated since it's a fluid. and why kinetic energy is more effective against shield is because it is a much more pinpointed pressure on the shield, as opposed to explosions, which is a shockwave across a larger surface area.
anyway, i digress, back to topic, i always think of hexagonal shields, like the one in the nanosuit in crysis. or something like that. i just think the shield might need more details that just a gradient fade,or even some different visual effects for different type of damage applied to the shield.