The $13 million Greville Emerald Kokoshnik tiara has belonged to the British royal family for decades, but is rarely worn.

Emerald Expositions Events Inc. in San Juan Capistrano is slated to raise about $300 million in its initial public offering, according to documents filed with the Securities and Exchange Commission.

Emeralds have, for centuries, captivated empires, royals, and the generally well-heeled and powerful. Egyptian queen Cleopatra was so enamored with the stone that she would carve her likeness into the ...