It is a crystal of sulfur. Such crystals conduct heat so poorly that the warmth from my hand would cause it to crack. Sulfur is one of the few nonmetallic elements to occur naturally in its native or pure form. Sulfur melts at 113 degrees Celsius, just above the boiling point of water, and burns at 270 degrees Celsius, producing a characteristic blue flame and choking fumes of sulfur dioxide. It's normally bright yellow, but impurities can color it gray, red or green.
Crystals are usually bipyramids or thick tabular crystals.
Sulfur is the ninth most abundant element in Earth's crust. Besides its native form, it occurs as a constituent of many common minerals. Sulfur is an ingredient in many things, including medicines, plastics, soaps, explosives, insecticides, paints, paper, rubber and fertilizers. More than 80 percent of its commercial employment is in the manufacture of sulfuric acid, which is used by almost every industry, from petroleum refining to cosmetics. Sulfur occurs in many places around the world.