Abstract: A glass-ceramic composition which is is crystallized in situ from a thermally crystallizable mixture containing from about 40 to about 80 weight percent of electric arc furnace dust and from about 10 to about 40 weight percent of glass. The composition has a Vickers hardness of at least about 5.5 gigaPascals, a Vickers fracture toughness of from about 0.6 to about 1.5 megaPascals.meters.sup.0.5, and a density of from about 2.8 to about 4.1 grams per cubic centimeter; and it contains from about 20 to about 40 weight percent of an iron oxide. When the composition is tested in accordance with the Environmental Protection Agency's Toxicity Characteristic Leaching Procedure, it produces a leachate which contains no detectable mercury, no detectable barium, no detectable selenium, less than 0.2 parts per million of detectable lead, less than 0.5 parts per million of detectable chromium, less than 0.1 parts per million of detectable cadmium, less than 0.005 parts per million of detectable arsenic, and less than 0.