Mineral-insulated cables for resistance thermometers consist of one or more copper wires that are embedded in highly-compacted magnesium oxide and sheathed in casing tube made from, for example, 1.4571 stainless steel.  For thermocouples, instead of copper wires, thermocouple cables suitable for the thermocouple type are used.  The most common standard sheath material for thermocouples is Inconel 2.4816.