Sci-fi: man has nanobots in him, and uses then to make diamond to help fund defence against interdimensional invasion

[troy]

As the title mentions, a sci-fi book where the main character somehow gets nanobots in him that he can control. At one point he bought a used up diamond mine and fabricates diamonds.

There is an inter-dimensional invasion attempt, but the portal is microscopic at the start.

Late 80s or in the 90s.

Thanks!