Soulcasting is the art of changing one substance to another. It is a form of Surgebinding. Soulcaster is a colloquial name for both persons who possess soulcasting ability and a type of fabrial that allows anyone to soulcast.
Nine out of ten Soulcasters were capable of a few limited transformations: creating water or grain from stone, forming bland, single-roomed rock buildings out of air or cloth. 
Soulcasting is used for a variety of purposes including food production, construction and repair, healing, and killing. Soulcastings typically happen at night, and under strict guard to keep the holy rite from being witnessed by anyone other than Ardents or very high-ranking lighteyes. Almost all Soulcasting individuals are members of the Ardentia and need a fabrial to Soulcast. (This is just one reason why Kabsal sought to kill Jasnah, the heretic, in effort to remove her fabrial from her.)
King Elhokar charges his Highprinces to use his Soulcasters to feed and house their soldiers. Most of the food and barracks at the warcamps on the Shattered Plains were Soulcast. Bridgemen often collect rocks outside of their warcamp so that Soulcasters can Soulcast them into food.
Soulcasting fabrials can be attuned to particular Essences, but they can theoretically work with all of the Essences.
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Q. Is Soulcasting mass-conservative (Soulcast a 1kg goblet, you get 1 kg of blood.)?
A. In most circumstances, yes.
A. 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 Kaladin. So you've kind of got it reversed a little bit though; Soulcasting has this additional restriction that the other ones don't.
If Stormlight for soulcasting need to be taken from specific gem, then I think dim garnet which Shallan had was enough, because Jasnah has always Stormlight with her, so she could just infuse garnet and then Soulcast Shallan's blood.