The Elf Queen wears a crown of three parts: black amethyst and obsidian for the dark elves, green emerald and flowering plants for the wood elves, and diamond and force magic for the high elves. When the elves were truly unified, they referred to themselves as the three branches of the elves, but since the war with the dwarves, it became customary to refer to themselves as the three Shards of the Crown.
Even though her crown remains whole, the Queen herself uses the term. At first, she used it sardonically, chiding her people for their animosities. But words used sarcastically have a way of sticking if they’re true, and the shorthand method of referring to one of the three branches of the elves is to call them shards.