Khuzdul

"Dark is the water of Kheled-zâram, and cold are the springs of Kibil-nâla, and fair were the many-pillared halls of Khazad-Dûm in Elder Days before the fall of mighty kings beneath the stone." ~ Galadriel to Gimli

At this Gimli looked into the Lady's eyes, and saw there not an enemy, but love and understanding. Such was the power of Khuzdul to a Dwarf.

Khuzdul, in Middle-earth, was always perceived to be a harsh-sounding language, which was used only by the Dwarves. This being that they were extremely secretive, and for their own reasons, did not wish to reveal their language. This goes all the way back to the Elder Days when Aulë created the Seven Fathers of the Dwarves, and a language for them. Little did that speech change over three ages of Arda.

Yet few knew the tongue, save Dwarves themselves. As said above, they would not reveal the secrets of their tongue, and taught it to even fewer, and when they did do so, they did it unwillingly. Very few Elves were allowed in their circle, and even then, most elvish philologists avoided Khuzdul, for to them, it was an unpleasant language. Curufin, son of Fëanor, was an Elf who had friendship with the Dwarves - it was through him that lore masters knew about Dwarves and their language. At least a few words made their way into the elvish tongues; Khuzdul kheled "glass" became Sindarin heledh, Khuzdul Khazâd "Dwarf", became Quenya Casar and Sindarin Hadhod, and vice versa - Khuzdul kibil "silver" must be from Sindarin celeb.

The Dwarves' secrecy of their language extended to their names - which were never revealed. Their real names were so secret not one of Dwarvish race could know it - Dwarves all took mannish names. However, Petty-Dwarves were an exception - Mîm told his and his sons' Dwarvish names readily to Túrin. Perhaps this was why Petty-Dwarves were hated by other dwarves.

However, this secrecy did not extend to places - Gimli readily told the Fellowship the Khuzdul names of the mountains over Moria. In fact, if others knew Dwarvish place-names, the Dwarves themselves would probably be very pleased. This is justified by Gimli's revering and friendly reaction towards Galadriel when she spoke Mirrormere's name in Khuzdul.

Not a lot of Khuzdul is known, only a few words and many guesses - perhaps Tolkien, like the Dwarves themselves, was unwilling to reveal the language!