Security features, e.g., reflective security features, and processes for forming security features are described. The security features comprise crystalline metal-containing particles having a primary particle size of from about 10 nanometers to less than 500 nanometers and including a continuous or non-continuous coating of a ceramic material. Inks comprising such crystalline metal-containing particles are also described. The crystalline metal-containing particles are preferably produced by flame spraying.