This is the 8945th most frequent Russian word.
рудн
It seems "рудн" may be a fragment or a typo. If you mean "РУДН," it refers to the "Peoples' Friendship University of Russia" (Российский университет дружбы народов).
The term 'рудная' in this context refers metaphorically to 'rudn' (ore) to describe the heavy and raw sensation of pain, drawing a poetic representation.
Когда он упал на камни, он почувствовал рудную боль.
When he fell on the rocks, he felt an ore-like pain.
Here, 'рудные месторождения' clearly refers to geological ore deposits, denoting materials mined for metals.
Девушка изучает геологию и интересуется рудными месторождениями.
The girl studies geology and is interested in ore deposits.
This analogy uses 'рудный' (ore-like) to poetically describe the brightness of the eyes, comparing it to the shine of an ore mineral.