a transparent ceramic
Ceramics are usually opaque and chalky-white — a china plate, a piece of bone, a bathroom tile. A transparent ceramic breaks that expectation: it is a polycrystalline ceramic, made from powder and fired like any other, that you can nonetheless see through almost like glass. The reason ordinary ceramics are opaque is not their chemistry but their microstructure: light bouncing off internal pores and grain boundaries is scattered in every direction, just as fog scatters headlight beams. Clear away those scattering sites and the same material turns transparent — the fog lifts.
There are two routes to a see-through ceramic. The first is to choose a material with a cubic crystal structure, such as YAG, spinel (MgAl2O4), or cubic yttria-stabilized zirconia; a cubic crystal is optically isotropic, so light crossing from one grain to the next is not bent (there is no birefringence) and grain boundaries barely scatter. The second route, needed for non-cubic materials, is to shrink the grains and pores below the wavelength of light so they cannot scatter it. Either way the dominant enemy is porosity: because a pore and the solid have wildly different refractive indices, even a whisper of residual porosity scatters strongly, so a good transparent ceramic must be fired to well beyond 99.9 percent density — residual porosity below about 0.01 percent — usually with ultra-pure powders, sintering aids, and pressure-assisted firing such as vacuum sintering or hot isostatic pressing.
Once you have it, a transparent ceramic can do things glass and single crystals cannot. It can be made much larger and in more complex shapes than a grown single crystal, and doped more freely — so transparent Nd:YAG ceramic now rivals single-crystal YAG as a laser gain medium. Transparent ceramics serve as transparent armour, high-pressure lamp envelopes, infrared windows and missile domes, and scintillators. The honest limit is how unforgiving the porosity requirement is: a single thin layer of pores, or one contaminated batch of powder, turns a would-be window back into a white opaque disc, which is why transparent ceramics are among the hardest of all ceramics to make well.
A polycrystalline Nd:YAG laser ceramic is fired until fewer than one pore in ten thousand volume-parts remains, and because YAG is cubic its countless grain boundaries barely scatter light. The finished disc is clear enough to serve as the heart of a high-power laser, yet it was made from powder, not grown as a single crystal.
Transparency in a ceramic is won by killing pores and taming grain boundaries, not by changing the chemistry.
A widespread misconception is that transparency needs a fancy composition. It is really a microstructural achievement: the same alumina that is opaque as a normal ceramic becomes translucent when its pores are removed. Chemistry chooses which route (cubic or sub-wavelength) is available, but pore-free density is what actually lets the light through.