What is the meaning of 刚 in 变形金刚? I came across the word 变形金刚 in a short children's story that I read and Pleco dictionary states that it refers to either shape-shifting warriors protecting the Buddha, or Transformers (which Google displays almost exclusively if you search the term).  Obviously the connection between the 2 meanings is clear.

But I'm unsure what is the meaning of 刚 in this phrase.  I asked my wife but she even believed the character is 钢 so didn't have a suitable answer.

Is there a historic meaning of 刚, or a historical transformation (!) from another character to this current one, which would enlighten me as to why this character is used in this phrase?
「剛」originally meant unyielding/stubborn/strong. 金剛 is a traditional phrase which is also used to translate a Buddhist word from Sanskrit (वज्र, vajra).

Vajra (PIE root: *weǵ-, English descendants: watch, wake, vigor, vigil) is seen by Buddhists as variously a thunderbolt or diamond, symbolising extreme hardness/power/indestructibility, and by semantic extension a kind of symbol or weapon, and further extension a guardian (spirit) which holds this weapon.

As you’ve already seen, 變形金剛 is a reference to Buddhist temple guardian statues (see 四大金剛) which each hold a kind of vajra (the weapon).