Here is a shot of some of the colors of dyed magnesite from my Flickr pages. This material is often sold as "chalk turquoise," "faux turquoise" or dyed "turquoise without the copper." The black "matrix" is black dye (sometimes simply black shoe polish) tmbled into the crevices of the irregular nuggets.

Turquoise miners and processors refer to soft, pale colored genuine turquoise that must be stabilized for use in jewelry as "chalk turquoise." Some sellers adopted than name for dyed magnesite even though turquoise and magnesite are unrelated minerals.