However, when Garrick watched his father, Gervaise Ollivander, struggle with substandard wand cores such as Kneazle whiskers, Dittany stalks, or Kelpie manes, he decided to devote his time to studying the finest wand cores. Before Garrick Ollivander's practice of using only the three " Supreme Cores", it was common for witches and wizards to present a wandmaker with a magical substance to which they had a particular attachment for use as a wand core. Ollivander's shop in Diagon Alley had changed the way that wands were created. However, when they were used in conflict with another, the connecting spells created Priori Incantatem, as seen when the wands of Harry Potter and Lord Voldemort, which both had cores comprised of a feather from Fawkes, Albus Dumbledore's phoenix, connected. When Chadwick and Webster Boot used them as a team, their wands with cores from the same Horned Serpent increased in power tenfold. When more than one wand had twin cores - meaning they shared the same material from the same creature - they were known as brothers and held greater power when used together. It could only be mastered by a witch or wizard who was able to face death, which was why Voldemort struggled to master the Elder Wand.
Thestral tail hair was also an interesting core - the wand couldn't simply be mastered by winning them.
For example, the wand-maker Garrick Ollivander would not use Veela hair for cores because he believed they made for "temperamental" wands. The core type was known to influence the temperament of the wand produced with it.