Diamond shapes are common used in flow diagrams, and can be easily created in Inkscape starting with a standard rectangle.
Select the rectangle tool.
Hold down the Ctrl key and drag to create a square.
ADDITIONAL INFORMATION:
Holding down the Ctrl key while creating the shape forces it to a square.
To convert a square to a triangle, select the Selector tool, then click the square twice to get the rounded arrows, and rotate while holding down the Ctrl key.
