Rarity is somewhat used to silly rambling, she is best friends with Pinkie Pie after all. And that girl is the queen of strange ramblings. So Dave is in the clear on that, even if she's giving him the slightest of confused looks at that babbling of his.

And good thing Dave can keep up the facade of being calm and collected. Rarity is doing her best to keep calm and hey! Him asking what she can do is a good way to get her to focus on it.

"Ah yes, that," it's now her turn to sound calm and collected, and look a bit smug. "I can make this diamond like gem as a shield, it's very effective and I've used it to save myself and my friends before."

Against a magic crazy plant lady which is a different beast from this. But she's not going to mention that.