This card is a cool idea. It could be used to protect your creatures, hinder your opponent’s creatures, or buff your Gnome Rider army.
This card’s targeting burrow ability is balanced. It has a similar effect to Freeze, but allows the creature to still attack (with halved attack). Compared to Arctic Squid, this card has a lower cost but it makes sense because of the above reason. Earth also lacks some reusable form of creature control (except if you count the nymph), which many elements have (Arctic Squid, Maxwell’s Demon, etc). The ability could also be used on your own creatures to protect them from the opponent’s harmful cards. This allows for burrowing of creatures from elements other then Earth, which could be used to open up new strategies with decks featuring more than one element, especially some powerful duo decks with Earth and another element.
The only problem I see is the cumulative buff effect on the Gnome Riders/Gemfinders. With fractal, you could make a deck with Peridot Stones and Gnome Riders. Because of their low cost, these creatures are easily fractal-able and you could have 6+ Gnome Riders with 13 attack with 6 Peridot Stones. While this may or may not be balanced compared to other cards, there is a possibility for a fast OTK is out there, especially considering upgraded cards. Just something to keep in mind.