In "Tumor", by Anna Leahy, the author wrote a poem about her mother's cancer death, with the title "Gravity 2", after she had written a poem about her father's death, with the title "Gravity 1":
She falls with the weight of herself
and her unsteady state, a state of un-being.
A moment arrives when my mother can no longer hold up.
When gravity overcomes pressure,
the lungs can’t draw another breath.
The memory I have of my mother
glows like a white dwarf. She sometimes eludes
my mind’s eye, but even a worried stone
dropped from my extended hand to my heart
is so heavy, it travels thousands of miles an hour.
What does "worried stone" mean in this context?