Complex sentence: The cushion-cut stone is one of the world's largest and most famous sapphires (its size about that of an egg) that was cut from a crystal mined in Sri Lanka.
Split into simpler sentences: The cushion-cut stone is one of the world's largest and most famous sapphires. The gemstone is one of the world's largest sapphires (its size about that of an egg) that was cut from a crystal mined in Sri Lanka.