a ceramic insulator
Look at the top of a wooden power pole and you will see fat brown or white porcelain discs holding the high-voltage wire away from the metalwork; unscrew a car spark plug and the pale tapering body gripping the central electrode is a ceramic too. A ceramic insulator is a fired, non-metallic solid whose job is to refuse to carry electricity — to hold two conductors apart at a large voltage without letting current leak between them. It is the everyday face of one of ceramics' defining properties: most oxide ceramics are superb electrical insulators.
The reason is in the bonding. In a metal, some electrons roam freely through the whole crystal, which is why metals conduct. In an ionic-covalent ceramic like alumina (Al2O3), every electron is locked into a bond or a filled ion shell; to move one you must lift it across a wide energy gap (the band gap, about 9 electron-volts for alumina) that thermal energy at room temperature simply cannot bridge. The result is an enormous resistivity — roughly 10^12 to 10^16 ohm-centimetre for good insulating ceramics, against about 10^-6 for copper, a gap of some twenty powers of ten. What little conduction does occur is not electronic at all but ionic: stray impurity ions, especially small alkalis like Na+, can hop from defect to defect, so a ceramic insulator's leakage is set by its purity and rises steeply with temperature (the opposite of a metal, which conducts worse when hot).
This is why insulator ceramics are among the highest-tonnage advanced ceramics: high-purity alumina for spark-plug bodies (which must hold 20 to 40 kilovolts across a few millimetres of hot, dirty engine environment), for the substrates that carry microchips, and for power-line and switchgear insulators; steatite and cordierite for lower-cost electrical parts. An honest caveat: no insulator is perfect. Every one leaks a tiny current, breaks down if the field is pushed high enough, and loses resistance as it heats or if alkali impurities creep in — which is exactly why insulator grades are specified by purity, and why a cracked or contaminated spark-plug insulator misfires.
A spark-plug insulator is made of about 95 percent alumina; across the few millimetres between the central electrode and the earthed shell it must withstand tens of kilovolts every firing stroke, thousands of times a minute, while glowing hot and coated in combustion residue. Only a dense, high-purity, low-alkali ceramic survives that duty.
The insulator's job is negative — to do nothing, to carry no current — but doing nothing under 30 kilovolts at 700 degrees C is a demanding ceramic problem.
Insulating and low-loss are not the same thing. A ceramic can block direct current beautifully yet still waste energy as heat under alternating fields; the DC resistance and the AC dielectric loss are separate specifications.