Formal:
* Rhombus: This is the most accurate geometric term for a diamond shape.
* Lozenge: Similar to rhombus, but can also refer to a specific type of candy.
* Quadrilateral: This is a general term for any four-sided shape.
Informal:
* Diamond: This is the most common and widely understood term.
* Kite: This is a less formal way to describe a diamond shape.
* Gem: This emphasizes the brilliance and sparkle associated with diamonds.
Other:
* Rhomboid: A parallelogram with equal sides.
* Kite: A quadrilateral with two pairs of adjacent sides that are equal in length.
The best word to use will depend on the specific situation and the level of formality you require.