Answer:
To Willy, diamonds represent tangible wealth and both validation of one's labor ( and life) and the ability to pass material goods on to one's offspring, two things that Willy desperately craves. Correlatively, diamonds, the discovery of which made Ben a fortune, symbolize Willy's failure as a salesman.
Seeds represent for Willy the opportunity to prove the worth of his labor., both as a salesman and a father. His desperate, nocturnal attempt to grow vegetables signifies his shame about barely feeding his family and nothing to leave his children when he passes. The seeds also symbolize Willy's sense of failure with Biff. Willy considers the American Dream infallible. His efforts to cultivate and nurture Biff went awry. Realizing that his all-american football star has turned into a lazy bum. Willy takes his son's failure and lack of ambition as a reflection on his abilities as a father.