In geometry, a diamond is 4 sided and so it is not clear what you mean by an "8 sided diamond". In jewellery, a diamond has faces or facets, not sides and, in any case, a diamond with eight facets is a very crudely cut stone.
(number of sides -2)*180 = sum of interior angles
1260
A 9 sided nonagon has 9 interior angles that add up to 1260 degrees
A mathematical diamond is a quadrilateral with four sides and four angles.
A 13 sided polygon also called a triskaidecagon or tridecaogn has a total 1980 degrees for its interior angles.
A rhombus is a 4 sided quadrilateral and its 4 interior angles add up to 360 degrees
A 10 sided decagon polygon has 10 interior angles that add up to 1440 degrees.
A 10 sided decagon polygon has 10 interior angles that add up to 1440 degrees.
There are 20 interior angles and they add up to 3240 degrees
30 sides
A 6 sided shape (hexagon) would have 6 interior angles of 120 degrees (assuming it is regular)
A regular six-sided hexagon has no interior acute angles, but it does have six interior obtuse angles, each measuring 120 degrees