The correct answers are B. eroded from and E. remains at.

Explanation
- Blank (i): The sentence tells us that the alluvial diamonds were found in "gravel beds" and were "carried hundreds of kilometers by ancient rivers." For rivers to carry these diamonds out of the "kimberlite pipes of the continental interior," the diamonds must have first been broken down or worn away from their source. Eroded from perfectly captures this geological process of wearing away and transport. While transported from is factually true, eroded from is the logically and geologically precise step that allowed them to enter the river system in the first place.
- Blank (ii): The sentence sets up a sharp contrast using "however" and "represent a very different category." Unlike the alluvial diamonds that traveled hundreds of kilometers away, these gem-quality stones are "mined directly from the volcanic matrix in which it crystallized." Therefore, this material remains at its geologic origin.

Difficulty Level
Medium-High. This question tests both scientific reading comprehension and the ability to track a clear geographic/geologic contrast between material that traveled far away versus material that stayed in place.