What is the effect of reciting here the particular images of gold mines, oil wells, and diamonds?
"Still I Rise" by Maya Angelou is a poem about racial oppression and poverty and the way that the human spirit nonetheless triumphs over external circumstances. The poem is written in the first person, addressed as a speech to a generic "you" standing in for the forces that oppress black people, in particular white and rich people who are conflated as oppressors.
The poem argues two things, first that the "I" of the poem will triumph over the forces of oppression and second that despite material deprivation and lack of power, the narrator (and the oppressed in general) still possess the same strength and indomitable will as their oppressors.
The three images of gold mines, oil wells, and diamonds are intended to be emblems of great wealth. Especially in the 1960s when this poem was written, these three materials were extracted from areas in the Middle east and Africa, with many of the profits from the mines and oil wells going to multinational corporations or local kleptocrats rather than the local people who lived on the land.