I would want to explore the imagery of this excellent poem in response to this question. What makes this poem such an accurate reflection of the experience of the body and mind going through grief is the kind of images that Dickinson employs to describe the impact of the grief on herself. We can see this in the first stanza when she describes her "nerves" as "sitting ceremonious like tombs." There is a sense of formality and ceremony in this image, but at the same time this is linked with grief and sadness as the nerves resemble tombs in the way they are presented.
The second stanza explores how grief impacts the body, and how "a quartz contentment, like a stone" descends on the body and its movements as this second stage of grief takes hold of the body. The use of the word "contenment" is an interesting one in this context, as it seems to suggest that there is something welcome about this gradual hardening of this leaden state that we enter in. Perhaps this refers to the permanence of such a state after the first volatile and emotional response to grief in the intial shock.
Lastly, the final stanza comments upon this stage of grief, calling it "the hour of lead." This again is another interesting image. We associate lead with something oppressively heavy and difficult to bear, which accurately refers to the grief that the speaker is enduring. However, the fact that it is called "the hour of lead" does lead us to believe that this is a stage and, just as hours pass, so this stage will pass to, and "the letting go" of the grief will come with time.