Hello!
To get the https://www.wowhead.com/item=77951/shadowy-gem, you need to collect a https://www.wowhead.com/item=77952/elementium-gem-cluster that drop from bosses and various objects, such as https://www.wowhead.com/object=210160/lesser-cache-of-the-aspects. The https://www.wowhead.com/item=77952/elementium-gem-cluster drop chance will be fixed after the server update.