What is the name of the thing heated by the sapphire crystallization furnace?

the crystals of alumina _ ＿Al2O3 are formed by crystallization in high-temperature aluminum-rich magma. Pure alumina crystals are called white corundum, colorless and transparent, with a hardness of 9. The crystals belong to a triangular system. Most of them are in the form of waist-drum hexagonal columns, and some are in the form of needles or plates. The aggregate is massive or columnar. Usually white, gray, yellow and other colors. If a small amount of Cr3+ is red, it is called ruby; If it contains a small amount of Ti4+ and Fe2+, it will be blue, named sapphire; Some crystals contain acicular rutile or other inclusions with six times symmetrical distribution, which can produce six-shot star awn, called star ruby or starlight sapphire, which are precious stones. Corundum is glass luster and has no cleavage, but there are often polycrystalline twins formed on the bottom or rhombic surface, which lead to cleavage. The specific gravity is 3.98. It occurs in igneous rocks and metamorphic rocks which are relatively poor in silicon, and is common in alluvial placers. North Carolina and Georgia in the United States, Urals in Russia and Transvaal in South Africa are the main producing areas.

There is no difference in chemical properties, but there are some differences in physical properties. As long as Cr3+ in ruby is in an excited state and emits red laser when photons with trigger frequency pass through, it can be used as a red laser source.