Rogue stone

A rogue stone (sometimes written roguestone ) was a small, extremely rare magical gemstone with a tendency to cause wild magic.

Description
Rogue stones were filled with rainbow colors that flowed as if in liquid, slowly swirling iridescently, when exposed to sunlight. They were extremely rare and seemed to repel each other&mdash;no two rogue stones were ever found in the same place at the same time. Single stones have been found in treasure hoards, cold regions, and sunk in swamps, but evidence about what type of rock they occurred in was inconclusive. Rogue stones were easily faceted along natural planes and the resulting faces gave the stones their iridescent property. A typical specimen had a base value of 5,000 gp  but was likely to be worth much more to wizards who knew of its uses. There were more than sixty known rogue stones throughout the Realms.

Powers
Rogue stones were used to cast the gemjump spell. Rogue stones did not have a detectable magic aura except at the time gemjump spell was being cast, and they were not destroyed or consumed by the initial enchantment or the activation of the spell.

The presence of a rogue stone within 20 ft (6.1 m) of a spell effect (either near the caster or the target, if any) increased the chances of that magic going wild or failing. At the recipient end, it decreased the chances of the target resisting or overcoming the magic effect, if applicable. If a rogue stone was used in the construction of a wand that produced a chaotic or teleportation effect, it increased the efficacy of that effect by either making it more difficult to resist or by increasing the level at which the effect was cast.

History
Rogue stones and gemjumping have played a role in several dramatic events in the history of the Realms.
 * Durlyn, the court mage of Brammas, Lord of Sulmarin, mounted a rogue stone on the regent's crown so he could appear before his liege in times of emergency. This served him well when thieves made off with the crown to their cavern hideaway. Durlyn gemjumped to the crown and vanquished the thieves by surprise and magic.
 * Faeros, a rogue mage, sold a rogue stone to an agent of the Red Wizards of Thay. After waiting a sufficient interval for the stone to make its way into the secret rooms of their Citadel, he gemjumped in, gathered up the rogue stone and all the spell books he found, and flew away.
 * Glemmora of the Rallyhorns cleverly hid a rogue stone and a spare spell book in the ruins of Myth Drannor and used gemjump to escape King Urgiathus of Dulgund where he had shackled her in the Dungeons of Torment.
 * Gulthas, the Black Mage, attempted to loot the hoard of Tiamat herself when she made her lair on the first layer of the Nine Hells known as Avernus. When discovered and pursued by three of her consorts, he gemjumped to the safety of his fortress on the Sword Coast near Baldur's Gate.
 * Heladar of Procampur sent a rogue stone as a gift to Princess Lustra, Royal Lady of Innarlith. One night he gemjumped into her boudoir and used sleep on her terrified handmaidens. He was able to spend the entire night wooing the sheltered princess before he was discovered. Thus began a passionate courtship, and they eventually married.
 * Laeral Silverhand once had a belt to which two rogue stones were affixed. One was attuned to Khelben Arunsun and the other to Alduth of Neverwinter. When she found the amulet belonging to the demon lord Arlgolcheir, she used sending spells to alert her allies, they gemjumped to her aid, and the trio summoned and defeated the demon.
 * Mercloak of Ruathym was able to exert his will after centuries of imprisonment in solid rock and escape his tomb. He appeared before his jailer, the wizard Ranthal of Berdusk, who had in the meantime become a lich, and obliterated him. He then proceeded to explore the Realms, much different from the Realms of his day, and revel in its pleasures. It was said he eventually returned to the spot of his enemy's defeat and spoke thanks to the truly dead mage for his trip through time to a greatly improved world.

Rumors & Legends
Some primitive humans believed rogue stones to be sentient and contained the conscious part of dragons or fabled heroes. Most sages agreed this view was strictly folklore.