From Dota 2 Wiki
Adding this gem to an item will enable that item to track games viewed for a league or team.
Spectator Gems track the number of games viewed in a tournament, league, or played by a professional player or team. Adding a Spectator Gem to an item will display the tracked stats in the item's description.
- Some items, such as Resonant Vibrance Set, require a certain amount of games viewed on its Spectator Gem to unlock additional styles.
- Special spectator gems can be acquired during certain events.