pokemon-go
So I just caught an Eevee. Now, I need 25 candies to evolve it. The fact is that in the GameBoy color versions, that Pokémon needed a stone to evolve instead of experience. Depending of the stone he was fed with (thunder, water or fire), he was evolving in a different Pokémon.
Is the stone feature also in Pokémon Go? If not, what decide the evolution of this Pokémon?
Bonus point : If it's possible, how can I get a stone?
Theoretically, yes, you are right that this new implementation could hinder your ability to catch rarer Pokemon.
However, this is a beta feature and Niantic is specifically looking for feedback like what you have posted above. While there is not a way to disable this new Nearby implementation currently, you should definitely submit this feedback to Niantic so they can take it into account before they introduce this feature universally.
Here is the link for submitting a bug report.
Based on the information provided in this thread, it appears that the silhouettes are due to the recent gender split in the Pokédex. If you don't have both genders of an evolved Pokémon, the evolution icon to that Pokémon will be a silhouette until you have gotten both.
In fact, looking through my own Pokédex, it even seems as though the silhouette is for any Pokémon you haven't caught/evolved since the update. For instance, I have 2 Sandslash, but both genders are shown as silhouettes:
Update if the gender isn't registered in your Pokedex then it will show a silhouette. Example male Dragonair shows Dragonite, female shows the Silhouette.
If you are looking at a MALE and have evolved a MALE - then the silhouette is now a colour image of the Pokemon
If you are looking at a FEMALE and have not evolved a FEMALE - then the silhouette is still a silhouette
or vice versa
Luckily, I had the perfect group of Sandshrew to test with (2 males, 1 female, and exactly 50 Sandshrew candy), so I evolved one of the males to see how it affected silhouettes. It appears that until you evolve one after the update, the evolution icon will display a silhouette. Furthermore, the silhouette is, indeed, gender-based.
Best Answer
Evolution stones as you know them (water, fire, etc.) don't exist in the current version of Pokémon GO. By default, when you evolve an Eevee in Pokémon GO, the evolution is chosen randomly from the first set of potential "eeveelutions" (Vaporeon, Jolteon, Flareon).
To get other Eeveelutions like Espeon/Umbreon, other requirements must be met. See @MBorg's answer for more details.
There is also a method for forcing a specific evolution by giving your Eevee a certain nickname before it evolves. See @Brant's answer for more details.