The poem uses various kinds of repetition to emphasize key words and images, and to create rhythm and musicality.
Some of these repetitions fall in quick succession, as in the anaphora of lines 5-6:
How a diamond comes into a knot of flame
How a sound comes into a word, coloured
Besides making the lines more rhythmic, this repetition underscores the parallel the speaker is drawing between the way "diamond[s]" come into being and the way feelings emerge through "word[s]."
There's also anaphora in the second stanza, although the repeated phrase is spread out more: lines 8, 16, and 21 all begin with "Some words." This repetition reminds the reader that all the imagery in these lines ultimately describes language itself.
The poem repeats other words and phrases as well. For instance, "black" appears toward the beginning of the poem, in line 2, and toward the end, in line 25. The phrase "From the earth's inside" also occurs at the beginning and end of the poem, in lines 3 and 25. Additionally, the simile about "a diamond com[ing] into a knot of flame" appears in the first and last stanzas (lines 5 and 24). These repetitions bookend the poem, so that the ending mirrors the beginning and the poem, satisfyingly, comes full circle.
The words "open," "word"/"words," and "diamond" also recur in each stanza, underlining the poem's central idea: that words can transform private experience into something beautiful and visible, like heat and pressure turning raw material "from the earth's inside" into diamonds. This process represents a kind of "open[ing]"—a word that suggests both vulnerability and transformation.