The line comes from the poem Still I Rise by Maya Angelou.
In the broader context of Angelou's work, the poem is about fighting against oppression; the "rising" is about human beings rising up against oppression into freedom. It is particularly focused on the rise of Black people in America out of slavery and toward equal rights, but also broadly represents all who struggle against oppression.
The specific line about diamonds is in the context of several other lines where Angelou mentions "gold" and "oil", suggesting that she intends to speak in general about what we economists call extractive resources---the sort of resource that a single company can come in and take to sell without the help of a broader society and government. These kinds of resources are systematically associated with corrupt and oppressive governments, because while they are extremely valuable on the market, unlike other sources of wealth such as land, factories, and an educated population, they can be easily taken and moved elsewhere and don't depend upon having a stable system of public policy to support them. This is called the resource curse.
It's unlikely that Maya Angelou had the details of this economic process in mind, but she intuitively sensed that resources such as gold, oil, and diamonds are systematically linked to oppression. She wouldn't use the example of corn or automobiles to make the point, because these aren't extractive commodities. Angelou describes herself (specifically implying her genitals and therefore her sexuality) as being this sort of commodity, which represents her as a thing of value---simultaneously valuable and treated as a thing to be possessed and sold by others, exactly the way slaves were treated. It may also be significant that gold, oil, and diamonds are all resources which are abundant in Africa, the continent from where the vast majority of slaves in the US were taken.
But the poem ends quite optimistically, repeating "I rise, I rise, I rise"---perhaps reflecting how civil rights have gradually but steadily improved over time, and Angelou's belief that they will continue to do so.