1) (in-universe) the mural is a stylized / symbolic image
As such, it is not "accurate to their appearances, in particular the location and shape of their gemstones".
The four diamonds are depicted having identical, rhombus-shaped gems that match the shapes composing the old Diamond Authority symbol, representing (just my guess) harmony, order, stability, ...
(image from Steven Universe Wikia)
But the real gems (not only Rose's one) are different:
(images from Steven Universe Wikia)
(image from Tumblr, could not find the original post)
2) (out-of-universe) the mural was based on old designs, subsequently revised.
Note that, apart from Yellow Diamond, all the diamonds' appearances are quite different from their final design (for example, Blue Diamond's hair is dark instead of white and lacks the "coil" around her neck).
While the authors had the general story in mind from the get-go, the character designs were not finalized until they actually had to appear in the episodes.
In a Tumblr post dated May 10th 2018, storyboarder Joe Johnston answered a very similar question:
Why does Pink's physical form not match her mural?
I drew those murals way before the Diamonds designs were finalized.
Yellow was pretty figured out because her appearance had at that point already been storyboarded.
Rebecca had done some very rough drawings of Blue, _____ and Pink and I was basing the murals off of those drawings.
When it came time to really design Pink, Rebecca chose to alter her hair and such, but not so far that the mural didn’t look completely wrong.
(the redacted name clearly is
which at that time hadn't been explicitly mentioned yet in the aired episodes)