The diamond of chastity is a phrase used to teach the youngsters of todays world the importance of a chaste relationship. A female diamond is the area (in the shape of a diamond) from her neck to her knees. A male diamond is in the area (once again in a shape of a diamond) from his belly button to his knees. As the saying goes, "Do not touch the diamond unless you are wearing a diamond," referring to, of course, being married.
The common hand motion of making a diamond in the air is highly understood as the international symbol for staying chaste.
Two girls riding home with one of the girls dad after a volleyball game:
Tatum: "Oh my god! Lisa is that girl that went to third base!"
Melody: "Lisa isn't as slutty as that girl Jen, she went all the way home!"
Dad: "Tatum do you remember what the shape of a baseball field is like?"
Tatum: *sigh* "Yes dad, the diamond."
Dad: "Thats right!"