How are level 21 Vaal skill gems created?

Hi,

Quick question .. I see a bunch of listings on poe.trade for level 21 Vaal skill gems, and I was wondering how these come to be?

As I understood it..

- Vaal skill gem drops are always Corrupted, so you can't use a Vaal Orb to turn a level 20 gem into level 21
- If you use a Vaal Orb on a normal level 20 skill gem, it could turn into the Vaal version of the skill or bump to level 21, but not both? And then it's Corrupted too.

Have I got one of those wrong? Or is there something else I'm missing?

There's a beastcrafting recipe that can create a level 21 gem.

Aha! Thanks for the answer!

That'd be

"

Level 21 Corrupted Gem

1x Rare The Sands
1x Rare The Deep
2x Rare The Deep with modifier "Craicic Presence"

..right?

Assume it's a random level 21 gem, which sometimes just happens to be a Vaal skill?