Well awktshualy...the definition says reflect, and while diamonds reflect internally, they refract light externally, so, the diamond is not bright because it reflects (those reflections are on the inside) they shine because they refract
But isn't their shine mostly due to the internal reflection rather than external refraction? That's why the gems are cut angularly, to help with the internal reflection.
Nothing is more on brand than a language pedant making grammar mistakes. What do you expect of person who thinks memorizing rules is the hallmark of intelligence?