The Burning Crystals are crystals found on Sunstrider Isle, to the west of the Sunspire in Eversong Woods. They have long been used to power the isle's experimentations. The mana wyrms were their guardians, until the Scourge invasion of Quel'Thalas drove them errant from the blood elves' lack of magical control over them.
The unchecked power of the Burning Crystals negatively influenced the isle's wildlife population and natural balance. For example, the nearby lynxes succumbed to the influence of the crystals, prompting the blood elves to put them down. The blood elves at the Sunspire will continue trying to rein in the Burning Crystals.
Although not called that, based on their appearance, they may be fel crystals. As these crystals have been present since before the Scourge sacking as noted above and would therefore not be powered by demonic energies, as the blood elves did not turn to using demonic energy until long after Arthas' attack. It is possible that Magistrix Erona may be lying about their nature. Another possibility is that this corruption could be attributed to the blood elves substituting the powering of these crystals with demonic fel energy, which may or may not be a secret kept from civilians, while the crystals themselves were there long before.