The Black Diamond is a modern waterfront extension to the Royal Danish Library's old building on Slotsholmen in central Copenhagen, Denmark. Its quasi-official nickname is a reference to its polished black granite cladding and irregular angles. Designed by Danish architects Schmidt Hammer Lassen, the Black Diamond was completed in 1999 as the first in a s…The Black Diamond is a modern waterfront extension to the Royal Danish Library's old building on Slotsholmen in central Copenhagen, Denmark. Its quasi-official nickname is a reference to its polished black granite cladding and irregular angles. Designed by Danish architects Schmidt Hammer Lassen, the Black Diamond was completed in 1999 as the first in a series of large-scale cultural buildings along Copenhagen's waterfront.