Nothing says "I love you" like a diamond.
Well, actually that's not true. A card that says "I love you" on it probably says "I love you" even more than a diamond because...you know, it actually says "I love you" on it.
And I guess a person saying "I love you" says "I love you" way more than a diamond because it is a person actually saying "I love you" which is much more clear and direct than giving someone some jewelry and letting them draw their own conclusions about your feelings for them.
But they say to show, not tell. And nothing shows that you love someone more than a fancy gift like a diamond. Except for maybe a really big billboard that says "I LOVE YOU" with a picture of the person's face on it, because that's the kind of gesture that shows that you love someone but also says "I LOVE YOU" on it.
But, short of a billboard, a diamond is the best way to show that you love someone. Maybe just get a card to go with the diamond, to make the sentiment explicitly clear.