The synthesis of the heavy elements occurs in successive layers. This contraction / dilation alternation is simultaneously the cause and the consequence of the fusion reactions. This is known as a pulsatory star.
At the heart of a massive star, the nucleosynthesis produces iron, the last stage of this type of reaction. At the heart of a star, of the type such as the Sun, the nucleosynthesis finishes with oxygen.
The iron of our own human body and the silica of the cliffs on Earth were produced in this way, by a generation of stars previous to the Sun.