How to i combine them and make a black soul gem.

You can't. Only thing you can do with soul gem fragments is to sell them for few coins.

Seriously thought i read somewhere 4 made a gem?

Nyet. The UESP wiki has no reference to this being possible. There is a lot of rumor and misinformation about it when googled, mostly people discussing it way back last November, when nobody knew much about the game yet. Some of those threads contained posts saying you could find NPC's to 'combine' the fragments into black soul gems, or whatever. But I've found them all to be false, just a bunch of hooey. They are vendor fodder, nothing more.

Although, there may be mods that allow them to be combined, haven't bothered to check.

Think there's at least one mod I've seen that allows you to combine them, but apart from that sell em or collect them or leave 'em where they are.

I collect soul gem fragments whenever I see them. In 275 hours of playing on one character, I have collected 22 soul gem fragments. There are 5 different types of soul gem fragments. According to the stats I have trapped 571 souls. Soul gem fragments are much rarer than soul gems.

Just doesnt make any sense... Why have them if u cant make a gem?

They looked to me like they are the bits of soul gem between the metal parts of the Black Star. That's what I presumed they were.

http://images.wikia.com/elderscrolls/images/d/d1/TESV_The_Black_Star_Crop.png

This could make for a powerful quest.
What if you needed to collect a certain number of fragments in order to combine them and make an incredibly powerful artifact?
Better yet - what if you needed to PREVENT that artifact from being made?

There's about a dozen long, deep storylines that could be developed just around this.

But more to the point:
I collect them, but haven't found a way to actually use them. For the moment, they appear to be utterly useless.

There just like many of the misc. items that have uses and purposes in the real world, but none in the game. It might be that Bethesda planned to do something with them, but forgot about it or gave up on it. As it is, they do nothing.

Arcadia the alchemist lady sold me a recipe that had Gourd as an ingredient. That made me pretty excited about collecting them. Then it turned about that the recipe was wrong and you can't even use Gourd for cooking either.

Maybe its part of a quest in the DLC.

Just wishful thinking.

Not everything in the world of Skyrim has to mean something. They're probably there, because the Necromancers practiced destroying soul gems for whatever necromancing purposes.

They make great decoration, why not chuck them into a kettle in your house?

Good ideA, will do that.