Notes to "Coal"
by Audre Lorde

Coal is black & is from inside the earth.  Writing as a black woman, Lorde compares herself to this coal:
I
is the total black, being spoken

from the earth's inside.
The second stanza considers words and their nature. Note how the last 2 relate to the first 2--they repeat it in reverse order.  This is called a chiasm.  The term is derived from the Greek letter X (chi) because they line up like a chi in a diagram.
 
 
A.     Some are diamonds flying free.
 

B.     Some are stapled wagers stuck in a book.

 X B'.    Some stick in her throat.
 

A'.    Some fly out.

In the third stanza, she asserts that just as the coal in the earth produces diamonds, so she in her blackness produces the diamonds of her poems.