Wand core

The core of a wand is a magical substance placed within the length of wood. The materials used for wand cores can vary widely, though certain wand-makers may prefer to use certain materials; for example, Ollivander only uses phoenix feathers, dragon heartstrings, and unicorn tail hairs. He will not use Veela hair as cores because he believes they make for "temperamental" wands.

If two wands have cores from the same source — such as the wands of Harry Potter and Lord Voldemort, which both had cores comprised of a feather from Fawkes, Albus Dumbledore's phoenix familiar — they are brothers. This can create the effect of Priori Incantatem if the wands come into conflict with one another.

It is worth noting that Harry's phoenix feather wand cost 7 galleons. Horace Slughorn, however stated that unicorn tail hair was worth 10 galleons a hair. It is possible that wands made with unicorn hairs are more expensive than those made with phoenix feathers. It is also possible, however that in the five years between the time Harry's wand was purchased and the night Slughorn made the comment that the market for wand material had fluctuated greatly. Very likely due in no small part to the capture of Ollivander by the Death Eaters.

Thestral hair probably makes for a very unusual core. As far as it is known, it has only ever been used to make a single wand - the centuries-old Elder Wand, the most powerful wand in existence. It would seem that the core being from a Thestral - which can only be seen by people who have seen death - has a connection to the effect of the wand being completely dispassionate about its allegiance, showing no emotion towards the wizard.

Behind the scenes

 * The Enchanted Feather Company is a company that presumably sells wand cores to wandmakers.
 * Peacock feathers may possibly be used as wand cores.