Misterski covered the basics. You want to increase the Spellpower related to the attacks you're using.
o https://ddowiki.com/page/Spell_Power...d_damage_types
The Warlock "blast" has 2 parts: the Eldritch blast itself, which increases automatically every few levels and is listed in the character advancement chart, plus the Pact damage, which also increases with Warlock level and also with certain Enhancements (typically Tainted Scholar tree*).
The default blast is Force, plus X-element for Pact damage component (so make sure you have that toggle turned ON - and see footnote*). Common wisdom is to grab the Utterdark Enhancement (TS tree), which turns that Force damage into Evil (aka Alignment) damage.
(This is all denoted in-game by the color of your blast. Turn your Pact damage off (toggle) to get a purple blast/aura, which is the default Force.)
o Force = Impact/Force/Kinetic/Impulse spell power
o Alignment = Light/Brilliance/Radiance spell power
o Pact = element varies, depending on chosen pact
Note that if you never turn Utterdark off, you can largely ignore Force for blasting. (However, Evard's Black Tentacles uses Force, so it's advised to fit it in at some point.)
Different Enhancements and damage spells use different spellpowers (but Enhancements most are Alignment, so that's covered). Check specifically.
If you can't find the spellpower you want/need, Potency is found on many items, and it covers all spellpowers. However, it typically is much smaller than a specific spellpower boost at the same level.
o Also, "Universal" spellpower stacks fully with all others. Some items, and many Enhancements, add Universal spellpower.
Don't forget the "critical" (aka Lore) half of any/all spellpowers. With all a warlock's blasting, these can trigger quite often during a battle and add up fairly consistently.
Also, keep an eye out for different types of sources - enhancement, equipment, profane, insightful, etc - those stack. Sources of the same type do not stack.
And the Spellcraft skill adds to all these (except Sonic (Fey Pact), which is Perform skill).
(* Footnote re Pact Damage - Some few builds ignore Pact damage (and most of the TS tree) and focus entirely on Eldritch blast. Pact damage gets a Saving Throw, Blast damage does not. This means that, to get the most out of your Pact damage portion of a blast, you want to increase your Evocation DC to make it harder for targets to save for half damage* against that part.
(* Or, for Pacts that allow a Reflex save (Fey & Celestial pacts), zero damage from Pact if that target has Evasion!)
Do not ignore your Pact damage unless the build you're following specifically directs you to do this - look at the build Enhancement plan and decide accordingly.)
All that said - what an Augment gem would do is to fit an extra Spellpower in to your gear tetris. Doesn't help the Critical part, but the base damage will thank you.