The poem's speaker begins the poem but considering what gift to get for a friend or loved one. She addresses this person directly, making the poem feel both intimate and conversational. Note how casual the poem's opening lines sound: the speaker makes it seem as though getting "Jewels" would be no big deal. She "could" obtain jewels if she "had a mind to"—that is, if she wanted to.
Apparently, the speaker isn't worried about cost. She dismisses the gift of jewels not because they'd be hard to get, but because her friend has "enough" already. Perhaps this friend is well-off and has no difficulty buying expensive things. Another jewel might just be one in a pile rather than something unique and special.
Notice the use of caesura in the first two lines:
I could bring You Jewels—had I a mind to—
But You have enough—of those—
Dickinson's poetry is filled with idiosyncratic dashes like these. Here, they slow the reader down. The caesuras create a halting, hesitant rhythm, subtly enacting the way the mind sometimes moves in jumps and starts. The dashes also make the poem feel less assured and more like the reader is witnessing someone's actual mind at work—that is, like the speaker is thinking through what to get this friend in real-time.
The poem is written in rough trochaic meter, meaning that it generally follows a stressed-unstressed rhythm (DA-dum). It uses a mix of pentameter, tetrameter, and trimeter, with odd lines being longer and even lines being shorter. Here are the first two lines scanned:
I could | bring You | Jewels—had | I a | mind to—
But You | have e- | nough of | those—
The second line is catalectic, meaning that it's missing the final unstressed foot. This allows it to land more emphatically on "those," as if the speaker is wryly poking fun at her friend's bounty of "Jewels."