Skill Gems are low lvl drops from monsters with the corresponding name.
You can only use Skill Gems for skills that you already have.
When you put the gem into a weapon you get a +1 lvl to that skill, even if the skill are at max lvl. This way you can get a lvl 15 skill up to lvl 16.
The gems do not stack, so using two of the same gem is pointless.
You use a Skill Gem by placing it in a socketed weapon. The Blacksmith can socket a weapon for you for a price.
Look at the item description of the weapon to see how many sockets it can have. Normally it’s 2.
For a price, the Blacksmith is also able to remove gems, but the skill gem will be destroyed when you do this.
This is an incomplete list, as I have only listed the ones I find most useful for my own build, but you can look up the rest here:
http://tosmain.ru/gems/
Archer Class Gems:
Multi shot: Vubbe Miner Gem
Oblique shot: Red Kepa Gem
Ranger Class Gems:
Barrage: Spion Archer Gem
Spiral Arrow: Ellom Gem
Fletcher Class Gems:
Crossfire: Pure Loftlem Gem
Barbed Arrow: Stoulet Gem
Magic Arrow: Lupusbunny Gem

