As >>16722131 points out, diamonds do make functional armor. As many other posters point out, they are not overly practical as large monolithic crystals for many reasons.

But there is ongoing R&D incorporating diamond powder with ceramics that has very high performance.
>Ceramics International Volume 50, Issue 3, Part A, 1 February 2024, Pages 4264-4273
>Ideal: A promising diamond-silicon carbide composite for enhanced ceramic armor
>https://www.sciencedirect.com/science/article/abs/pii/S0272884223031735
FWIW I think fiber laminate ceramic composites are overall better unless you need the absolute thinnest high-velocity rifle/frag protection regardless of cost.

>Ivanka no longer at the White House
>2nd term goes off the rails
really makes me think