If you are referring to the Sports-team, it is a name and most people would just say "Diamondbacks" although the last "d" would be sort of silent, because of Spanish pronunciation conventions.
If you literally wish to say "diamond backs" as in "backs" made up of "diamonds" (in the same way that "silk scarves" are "scarves" made up of "silk"), you can say "espaldas de diamantes".
vendedor del diamante
When you are talking about suits of cards, the word "diamante" is used.
"Real diamond" in Spanish is "diamante real". It is pronounced "dee-ah-MAHN-tay ray-AHL". Please see the Related link below for confirmation of the translation.
Arizona Diamond backs,
Midre Cummings
diamante
diamond
diamond
Diamante.
diamond
I say yes a diamond can be a trapezoid.
18 games with the Diamond Backs