Diamonds, geometrically (6)
I believe the answer is:
rhombi
'diamonds geometrically' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for rhombi that I've seen before include "Diamond shapes (maths.)" , "diamonds" , "certain figures in mathematics" , "Four-sided figures" , "Forms of parallelograms" .)