Crown of Durin

A group of seven stars mysteriously reflected from the surface of the lake of Kheled-zâram (also called the Mirrormere), even when the Sun was shining in the sky above the lake. In the ancient days of the First Age, they were glimpsed by Durin the Deathless, who took them as a sign to settle in the Misty Mountains that rose above the lake, and so Khazad-dûm was founded. The seven-starred crown became an emblem of the House of Durin, and was prominent among the inscriptions on the West-gate of Moria.

It's unclear whether the stars of the Crown of Durin represented a real constellation. Some sources suggest a link with the Plough or Big Dipper (on the basis that it is made up of seven prominent stars) but there's no direct evidence to support this connection. Another possibility would be the Northern Crown, Corona Borealis, which is also made up of seven stars, but in a curving crown-like chain. That would fit Gimli's description of 'gems upon a silver thread' rather better than the Plough, and also matches the crescent shape of stars shown on the Doors of Durin to some extent. It's also possible that Durin's Crown wasn't connected to any real stars at all, and that the Crown reflected by the Mirrormere was a unique effect unrelated to the actual stars of the sky.