when using Obsidian, if you don’t get one of the choices you are looking for, try to pick an affixes that you think you might want to use in another Build, so you already have an item with a Crystal affix.
using Quartz is the only way to remove a Crystal affix from an item. but this is a very bad idea. Quartz removes a random number of affixes, so if there is an affix you really don’t want to lose, then it is better to just use an Obsidian on another item.
if you don’t want to save your wasted Crystal Item, you can spend 500,000 Gold to convert it into 1 Ultra Rare Crystal. Obsidian is Ultra Rare, so you have a chance to get it back this way, or at least end up with another Ultra Rare Crystal.