When you get a Soulstone, it comes with once option. If you polish the Soulstone, you will be able to unclock 5 other options.
You need Gold and Soul Powder to Polish Soulstones. Soul Powder can be acquired from Raid. There is a chance that a Soulstone may fail to polish. When a Soulstone has been successfully polished, a new skill option is added to the Soulstone.
The option values from polishing Soulstones are assigned randomly. This is why adventurers try so hard to get the best results.