https://medium.com/the-nasa-psyche-mission-journey-to-a-metal-world/planning-to-measure-an-unknown-asteroid-c6b1b64b5d6e

We’ve got a leading hypothesis about the asteroid (16) Psyche: it is the metal core of a tiny planet that had its rocky exterior smashed off by some number of hit-and-run collisions, all in the first few millions of years after solids began to form in our solar system, long before the Earth was formed. And now Psyche orbits our Sun in the outer asteroid belt, the biggest bare metal object in our solar system.

Well, we think it’s metal, mostly nickel and iron. Several density estimates of Psyche have been made, including 4,500 ± 1400 kg m^-3 [1], 6,980 ± 580 kg m^-3 [2], 6,490 ± 2,940 kg m^-3 [3,4], and 7,600 ± 3,000 kg m^-3 [5]. These high-density estimates contrast strongly with the estimates for rocky asteroids: 1,380 kg m^-3 to 2,710 kg m^-3 for rocky asteroids, roughly one-third to one-half their parent-rock density of around 3,300 kg m^-3 [6]. (Here I am using “rock” for silicate rock, as opposed to metal.) Most asteroids, therefore, appear to be fractured or have otherwise high porosity, and so their bulk density is lower than their pure material density. And Psyche’s density estimates are higher than all the rocky asteroids.