what are pyrex containers made of?

Pyrex is a brand name for glass that is very tough & can handle extreme hot or cold.

Glass is a relatively fragile material. During manufacturing, to make it MORE durable, it must go from it's molten state to cool or cold over a long period of time. This relieves the stress within the glass. As an example: Little crystal glass figurines, unicorns, boats, ballerinas and so forth, that you see a glass sculpturer make at the fair or a little glass shop, cools down VERY rapidly in normal room temperature. THEY ARE VERY FRAGILE because they keep the stress internally from cooling so quickly. A Coke bottle that has it's neck heated and stretched out, may just suddenly break at a later date without even being touched.

Pyrex is cooled VERY slowly such as 12-48 hours. Because of that it is very tough and can stand relatively quick temperature changes and high temperatures. It's still not a good idea to take it from the oven and let it come in contact with cold water. It still might break.

Different formula that regular glass, more borax than regular glass( borosilicate ) and, annealing process.

Glass with a high boiling/melting point, I think.