Legendary Master Sunstone
The Legendary Master Sunstone is an enormous orange gemstone located on Rainbow Island where it is protected by Flame the Sunbird. According to Flame:
Almost immediately after the Sunstone was introduced in Sonichu, it was stolen by Black Sonichu, acting on orders from Giovanni. While making his getaway and evading Flame's pursuit, Black Sonichu interfered with the relaxation of Bubbles, Wild, Punchy and Angelica, causing them to meet for the first time. This in turn attracted the attention of Magi-Chan, who brought the Chaotic Combo together to confront Black Sonichu alongside Sonichu. The six Electric Hedgehog Pokémon squashed Blachu and returned the Sunstone to its proper place on Rainbow Island. Despite its apparent power and how easy it is to steal (Black Sonichu was only stopped because he coincidentally pissed off every other Electric Hedgehog in the world on his way home) the Legendary Master Sunstone is never mentioned again.
The Legendary Master Sunstone is a gigantic rip-off of the Master Emerald from Sonic & Knuckles. Additionally, its appearance, name, and color is based off the Sun Stone item from the Pokémon games.