I've 4 tables, ItemCategory, Items, DiamondCategoy and Diamonds.

The user store item details and specify whether or not it has a diamond on, for example:

ItemCategory: Ring
Item: R1
If there is a diamond then:
DiamondCategory: Round
Diamond: D1

So R1 of Ring has D1 of Round

An Item could have no diamonds, for example:

ItemCategory: Ring
Item: R1
DiamondCategory: None
Diamond: None

I can't figure out how to design the relationships. I came up with this solution, correct me if I'm wrong.

ItemCategory:
c_Id >> PK

Items:
p_Id >> PK
c_Id >> FK
d_Id >> FK

Diamonds:
d_Id >> PK
dc_Id >> FK

DiamondCategory:
dc_Id >> PK

Is that correct?.

A diamond can only be on one ring, but a ring can have multiple diamonds. Instead of having a diamond ID in the Items table put an item ID in the diamond table. Otherwise I think it looks right.