Soulcasting is a fairly unique form of Surgebinding, and seems to behave in a similar way to how a soulcasting fabrial works. Soulcasting surgebinders seem to be able to take the stormlight directly from gems for soulcasting without taking the stormlight into themselves. When this happens, the surgebinding behaves like the fabrial does; you take stormlight from specific colour gems based on what you are soulcasting into. See this "Word of Brandon" (emphasis mine):
I finally got to ask a question about the Stormlight Archive that Windrunner17 and Chaos helped me with which was: "Why Can Kaladin Surgebind with any gem type but Jasnah and Shallan need specific types?"
A lot of that will be explained as the series comes along. It is really the difference between Soulcasting and the other forms of Surgebinding. It's more a quirk of Soulcasting than it is something that is different about about Kaladin. So you've kind of got it reversed a little bit though; Soulcasting has this additional restriction that the other ones don't.
source (question 4)
It seems as though soulcasting is unique in surgebinding in that the stormlight can be directly fed into the object being soulcast from the gem, without being sucked in; and the gem colour affects what changes can be made.
As far as the cracking gem aspect goes, that part could be explained even if she were drawing stormlight into herself for the soulcasting. Gems shattering appears to be a consequence of taking too much stormlight out of a gem too quickly (along with the fact that the gems in circulation have imperfections). It's similar to cooling down something too quickly, you create a stress in the gem and it cracks. Or at least that seems to be the common accepted theory. Some discussion on gem cracking can be found here.
Now, in terms of pure speculation, it seems as though gems are acting as some sort of "key" in the soulcasting process. This reminds me a lot of other similar circumstances in the Cosmere. Metals on Scadrial are keyed to allow access to Preservation's power, and the type of metal changes how the investiture manifests. Aons in Elantris are keyed to access the Dor, and the Aon used changes how the investiture manifests. It stands to reason that by the same Cosmere logic, the gemstones may be acting as keys which allow the stormlight to change the objects, and they keying affects which changes are possible. Again though, that part is more speculation without any direct support.
Some additional discussion on Jasnah's soulcaster can be found here and here.