A many pointed star. It's clear and puts off a bright light as a constant. The light flashes brighter when the star is used.

Ancient relics. Enchanted by the wizards of old.

Possibly glass. Whatever it is it has been altered by magic to withstand the whither of time itself.

It has a good weight to it, but it isn't outrageously heavy.
Peter was able to pick up the magic star with little effort.

They were made many years ago.
Once upon a time there were many masters of magic. They were silver haired wizards who enchanted relics. They created a time star and a magic star. (It us unknown if there are any other stars of magic)

(Note: the use of these depend upon the skill level of the magician wielding it. Mage's are more in touch with the relics since there power comes from the magic star)
Time star-
Transport people through time
Change cycles of aging (I.e how long people live)
Show people the future or past
Magic star-
Grant people magic (Mages)
Arloch magic in three different ways (Disperse magic, block magic from the user, and give the magic to someone else)
Enchant relics (with the use of less magical abilities)

Background
Time star-
Made by the wizards.
Sometime before 437 it was acquired by lord Bren
After that it was presumably taken by lord Pond and locked away with all other items of magic around 439
Some point in the future it was most likely given to Peter's guild of magicians

Magic star-
Created by the wizards of old and gifted to the mages
Stayed with the mages at the Silver Star guild for an incredibly long time
Sometime between 437-439 lord Pond and Brooklyn took over the guild. They arloched and killed many mages and took the star away from them
Brooklyn kept it until 439 when Markus smashed it into pieces
After Brooklyn was arrested Peter and his friends took the shards with them to see if they could work together to operate it (As they had done to defeat Brooklyn)