I will show you guaranteed ways to get some amount of soul gems, depending on which you think about boring and fun. Happy reading!
Disclaimer: They ways aren't arranged by most trusted or fastest!

1. Mini games.

Well, first I will say that some people like them, but others don't. To earn fast soul gems from them, you will need first to master them! And when you master them, you may earn like 25-40 soul gems in 30 minutes from them (according to me and other people in the forum). Keep in mind - they are very hard, so you will need to make some skill in only 1 specific mini game.

A tip: Gamehag Katana is the most fun and easy mini game to learn soul gems. I personally earn 37-44 soul gems from it for only 30 minutes! There is no luck, only skill.

2. Daily login.

You can't earn many in this, but I will say some things you should know. The first is that if you login 7 days in a row, you will get a bonus 20 gems. So, let's do some calculations:

5x7 + 20 = 55 soul gems. That is the default you can earn like most users, but you can earn even more with "Mannaz" rune! If you have it activated, you will earn 7 soul gems daily.
7x7 + 20 = 69 soul gems weekly!

So, next time you get "Mannaz" rune from somewhere, don't sell it, selling it will get you only 5 soul gems, so activating it will get you more gems (14).

3. Leveling up.

I can't tell you exactly how much soul gems you will get for every level, but with every next level, the soul gems will be more. You will need XP to level up.

A tip: You can get XP by commenting on the forums or writing articles! But keep in mind - there is a limit on how much comments you will receive XP!

4. Writing articles.

Just like this one! If your article looks good (without spelling or other errors), you may get the max amout of soul gems which is 1000. Usually most people get 300 gems for 1 article, but if your article is long enough, not copied, doesn't have spelling, punctuation and typing errors, you may get 1000!

5. Getting in top 3 players.

If you get in this top after the day is finished, you will get soul gems!

1 - 1000 soul gems.
2 - 500 soul gems.
3 - 300 soul gems.

Although I mention this top here, it's actually close to impossible for normal users to be in this top (for some deep reasons)... The only way to get in this top is if you get +100 active referrals.

That's all the ways which guarantee fast soul gems. If I missed some trusted and fast way to earn soul gems, share it in comments! :)
Thank you for reading!