Pokemon Presents provided more insight into what we can expect when Pokemon Scarlet & Violet come out on November 18th. One of the big highlights was Pokemon turning into shiny gem versions of themselves during battle, which is known as the Terastal phenomenon.
During battle, you will be able to select the Terastallize option to turn your Pokemon into their crystal-like forms (similar to Mega Evolutions or Gigantamax Forms). Your character will hold up a stone, and the Pokemon will Terastallize, changing their appearance as the process completes. Terastallize will be available to all Pokemon native to the Paldea region, which already makes it more versatile than Mega Evolution in previous games.
Terastallizing your Pokemon will provide a boost to your Pokemon's type and strengthen their move power, in a manner similar to the Same Type Attack Bonus (STAB) that is present throughout Pokemon games. The Type will also overwrite a Pokemon's current type while it is Terastallized, which can also change its weaknesses mid-battle.
Pokemon can also change their type during Terastallizing, which is dependent on their Tera Type. For example, Pikachu is normally an Electric type, but it is possible for it to become a Flying Type when Terastallizing. Eevee was another example, being able to access Grass or Water typing with its Tera Type without having to evolve into Leafeon or Vaporeon. The Tera Type will also determine the appearance of a Pokemon, with Eevee sprouting flowers as a Grass Tera Type, but it has a crystalline spout as a Water Tera Type.
There wasn't any information on what part of a Pokemon determines their Tera Type, but it could be random like Individual Values (IVs) or dependent on the area that you capture them in. From the footage, you will need to obtain an item before the Terastal Phenomenon becomes available to you, similar to obtaining the Key Stone before Pokemon were able to Mega Evolve.
There will be Tera Types for every type currently available in Pokemon Scarlet & Violet, and you will be able to discover Pokemon with more Tera Types as you explore the region. But there is no doubt it will become a powerful tool when battling, and open up more potential strategies when traveling around the Paldea region.