Through the Legendary Dungeon in Shakes & Fidget you can now also get orange gems (or legendary gems). They are a lot better than the black gemstones, however they are also much more difficult to obtain.
This is How You get Legendary Gemstones
Whenever the legendary dungeon opens its gates, you can get new legendary items. For a legendary item you have to play through the dungeon completely once. Then you will receive a random legendary item, which belongs to the current set of the dungeon.
You can now find this item in your scrapbook under the "Legendaries" tab at the bottom. And here comes the in-game tooltip:
Legendaries do not count towards your scrapbook's fill level but increase the chance of Legendary gems in the gem mine!
And that's all. The more different Legendaries you have found while playing through the legendary dungeon, the higher the chance of legendary gems.
Legendary Gem Bonuses
While black gems bring the bonus to all attributes, the legendary gems will only improve your constitution and your main attribute. This makes them the ultimate weapon, because these are of course the two most important attributes.
So the second bonus on the main attribute is based on the class. For example, if you put your mage item on your companion (warrior), the gemstone changes its attribute from intelligence to strength.
The legendary gemstones add about 40% more attribute points to your main attribute and constitution compared to black gemstones.