taken from j.R.R. Tolkien's The Silmarillion:
"In that time were made those things that afterwards were most renowned of all the works of the elves... and at the end of all Feanor made the silmarils. As three great jewels they were in form, but not until the end shall it be known of what substance they were made. Like the crystal of diamonds it appeared,and yet was more strong than adamant, so that no violence could mar or break it...and the inner fire of the silmarils Feanor made of the blended light of the trees of valinor...therefore even in the darkness of the deepest treasury the silmarils of their own radiance shone like the stars of varda; and yet, as were they indeed living things, they rejoiced in light received it and gave it back in hues more marvellous than before. "