We used to have a 1kilowatt amplifier that we tested wattmeters with that had Beryllium oxide inside as part of the heatsink compound formula.
Wikipedia.
Beryllium oxide (BeO), also known as beryllia, is an inorganic compound with the formula BeO. This colourless solid is a notable electrical insulator with a higher thermal conductivity than any other non-metal except diamond, and actually exceeds that of some metals.[2] As an amorphous solid, beryllium oxide is white. Its high melting point leads to its use as a refractory.[3] It occurs in nature as the mineral bromellite. Historically and in materials science, beryllium oxide was called glucina or glucinium oxide.
Like all beryllium compounds, BeO is carcinogenic and may cause chronic beryllium disease. Once fired into solid form, it is safe to handle as long as it is not subjected to any machining that generates dust.[8] Beryllium oxide ceramic is not a hazardous waste under Federal law in the USA.