The Glittering Caves, or the Aglarond, as they were known, are the caverns at the back of Helm's Deep. They were originally worked by the Numenorians and later used as a refuge and for storage by the Rohirrim.
During the Battle of Helm's Deep, Gimli fought in these caves and discovered their great beauty. After the war he returned to them with other dwarves from Erebor, and he became Lord of the Glittering Caves. The new gates of Minas Tirith were forged there, and the dwarves continued to make many fine things for both the Gondorians and the Rohirrim.
Boring you can find textural reference with the maps to the Glittering Caves (Aglarond) on pages 46, 65, 88, and 133 in the revised edition of Karen Wynn Fonstad's The Atlas of Middle-earth. Val's bit about the forging of the replacement gates out of mithril and steel for Minas Tirith can be found on the last couple of text pages of Appendix A in RotK where it finishes up Gimli's part in the tale.
OK thanx o you both and the stupiest thing is i've got that atlas to and have been browsing and reading through it a while, while rereading the sil