Suggestion(s) for the shield,

• 99% Blocking.
• A Strong thorns effect. (reflecting 75% of damage done, which is always active).

While granted with how OP the gem armor is, the 99% blocking is probably only useful before you upgrade from Red Matter to Gem and/or if the shield is the first red matter item you make, but the thorns would help with clearing out hordes of opponents attacking as well as bosses, especially if it's reflecting a percentage of damage being dealt.

I had a similar idea with thorns based on charge level costing emc. For an active ability a knock-back based on charge level, possibly deflecting projectiles in range as well.