Mermaids in my setting have the power to manipulate shells. In more specific terms, they can alter a shell's shape as if they are made of putty, as well as fuse them seamlessly. This enables them to make weapons and armor out of seashells, but then this brings up an interesting exploit.
You see, thanks to this question, I know limpet teeth is the hardest known biological substance, and is also among the strongest. I also see that abalone shell would be good for armor.
Given that mermaids use seashells to form structures, as well as to create weapons for self-defense, it makes sense they'd discover limpet teeth (which I'll handwave as counting as 'seashell' for the purposes of being manipulated by mermaid powers) and abalone shell and utilize them.
For example, a mermaid with abalone shells could slap one on her shoulder, smooth it down, and have a pauldron, then continue to make herself abalone plate. She could also make a spearhead out of limpet teeth to stab hungry sharks with.
My question is, how useful would these specific seashells be for self-defense? Specifically, if a mermaid were to use these seashells to form armor or weaponry in order to defend herself, how useful would they be?
These mermaids would be defending themselves against sharks, crazed knights, pirates, other mermaids, giant octopi, and the occasional demented and/or obsessive sailor. As for tech level, think mid-medieval.