The pegasian crystal is an add-on dropped by Cerberus. When used with a pair of ranger boots, Pegasian boots will be created. This process cannot be reversed, and requires level 60 in Runecraft and Magic (boosts cannot be used), granting 200 experience in both skills.
The crystal is inexpensive compared to the other crystals dropped by Cerberus. This is due to the small supply of Ranger boots available in the game.
For an exhaustive list of all known sources for this item, see here
A player uses the crystal on the pair of boots, upgrading it.