Seemingly unique to Blackreach, the Geode Vein has a glowing blue appearance.
When mined, it has the possibility of producing the following:
Certain Geode Veins will only produce Corundum Ore while others will only produce Soul Gems. The grade of the Soul Gem is directly dependent on your level. For example, at level 4, Soul Gem producing veins will only output petty soul gems and precious gems. At level 50, any grade including black can be mined. Black soul gems can be mined at levels as low as 18. Some Geode Veins provide Corundum Ore, while at least one Corundum Ore Vein (near the Mzark Tower exit, at a Falmer camp) provides Soul Gems. Possibly the Geodes themselves and their contents got mixed up unintentionally during development.
There are at least 19 veins in Blackreach. Six of these seem to only give Corundum Ore. A few sublocations also yield this ore.
They can also be found in various locations in The Elder Scrolls V: Dragonborn.