Epic gems in Dragon soul patch are obtained through 2 currencies, either mote of darkness which drops from every boss or an essence of corrupted Deathwing from Deathwing only.
Mote of darkness drops on every boss and let you buy a crystalline geode( which can drop uncommon, rare and a small chance at an epic gem). While the 2nd option elementium-coated geode that you buy with essence of corrupted Deathwing only drops epic gems but is still rng and is restricted to one weekly as you can only kill Deathwing once weekly on a character. I feel this is way too slow and rng for a progression which should be much faster considering the faster pace of cataclysm classic.
It would be interesting and more fun to either let you buy an elementium-coated geode with mote of darkness for maybe 2-4 of them and/or to add epic gems from prospecting as it was done in wotlk and worked well.