Yellow Sapphire is a stone of Jupiter(Guru) and it is meant to strengthen some aspects of life like:
Ease of Finding a Partner
Having a smooth Relationship
Stability in Financial Status
Growth in Career
Diamonds are stones that signify Venus(Shukra) that is meant to strengthen some aspects of life like:
Materialistic Pleasure
Ability to meet & carry on a decent to a good life in a partnership
Having said that, Diamonds(like Blue Sapphire) do not suit each and every person. This is not the case with yellow sapphire. Jupiter is auspicious even when it is weak or debilitated.
Both the stones are meant to address different facets of your life depending on their placement and dignity. Having said that, it is perfectly fine to go ahead and wear them as you deem fit.
