a glass-ceramic
A glass-ceramic is the best of two worlds fused into one material. You make it as a glass, so you get to use every easy, precise glass-forming trick, melting, casting, pressing, blowing, into a perfect shape. Then you crystallize it on purpose with a careful heat treatment, converting most of that glass into a dense, fine-grained crystalline ceramic, so the finished part gets the strength, toughness, and heat resistance of a crystal. It is a ceramic you can shape like a glass, which is a genuinely clever piece of engineering.
The trick is controlled crystallization, and it hinges on doing devitrification deliberately and well. The secret ingredient is a nucleating agent, a small addition such as titania TiO2, zirconia ZrO2, or platinum, that seeds millions of tiny crystal nuclei evenly throughout the glass. The heat treatment then runs in two stages: first a lower-temperature hold at which those nuclei form in huge numbers, then a higher-temperature hold at which crystals grow on every nucleus at once. Because there are so many nuclei, no single crystal can grow large; the result is an extremely fine microstructure, often with grains well under a micrometre and more than 90 percent crystalline, far finer and stronger than the coarse, sparse crystals of accidental devitrification.
By choosing the composition and the crystal phase, engineers dial in remarkable properties. The famous lithium-aluminosilicate (LAS) glass-ceramics grow a crystal that shrinks slightly on heating, which cancels the glass's expansion to give a material with a near-zero thermal-expansion coefficient, immune to thermal shock. That is the science behind glass-ceramic cooktops and stovetops that take a red-hot pan and a splash of cold water without flinching, telescope mirror blanks like Zerodur that hold their shape as temperature drifts, missile radomes, and even machinable grades like Macor that can be drilled and turned like metal. A glass-ceramic is proof that controlled crystallization can turn a glass's worst enemy into its greatest asset.
A black glass-ceramic cooktop is the everyday showcase. It is cast and finished as a smooth glass sheet, then crystallized into a near-zero-expansion ceramic. That is why a burner can glow red beneath one spot while the rest stays cool, and why a boil-over of cold water hits the hot surface with no crack, the two temperatures try to make different parts expand by different amounts, but a near-zero expansion means there is almost nothing to mismatch.
Formed as a glass then crystallized to a fine-grained ceramic, a glass-ceramic can have near-zero expansion and superb thermal-shock resistance.
A glass-ceramic is mostly crystalline, not glassy, even though it started as a glass and often still contains a little residual glass between the grains. Its properties come from the crystals grown inside it, not from any leftover glass.