What does "rob it of a few petrified spars" mean in "Pride and Prejudice"?

Question

In Jane Austen's Pride and Prejudice, when the idea of visiting Derbyshire is first brought up to Elizabeth, she immediately thinks of Pemberley and Mr. Darcy, who lives there, and then makes a comment about it:

Elizabeth was excessively disappointed; she had set her heart on seeing the Lakes, and still thought there might have been time enough. But it was her business to be satisfied—and certainly her temper to be happy; and all was soon right again.
With the mention of Derbyshire there were many ideas connected. It was impossible for her to see the word without thinking of Pemberley and its owner. 'But surely,' said she, 'I may enter his county with impunity, and rob it of a few petrified spars without his perceiving me.'
The period of expectation was now doubled. Four weeks were to pass away before her uncle and aunt's arrival.
_{Pride and Prejudice, chapter 42}

I'm having trouble understanding what "rob it of a few petrified spars" means here. In context, it seems to mean something along the lines of "do some sightseeing", but I'm sure there's more nuance and context that I'm missing.

What does this phrase mean? Where does it come from, and what does Elizabeth mean by it here?

Answer 1

This 2001 edition of the book edited by Robert Irvine has the footnote:

Fluor-spar, a beautiful mineral known as Derbyshire Spar or Blue John Stone, is still mined from caves at Castleton in that county; or Elizabeth may have in mind the petrified objects produced by the hot mineral-springs at Matlock.

So it's not an idiom: Elizabeth is just saying that she'd "rob" (bring home) a few "petrified spars" (hunks of 'Blue John', or perhaps a trinket made from the material).