I was making a wizard, and I need a diamond, or diamond dust. Am I able to use creation to create a giant chunk of rock and then use that chunk to make a (probably rough) diamond? I know I don't have Jeweler's Tools, but I'm more looking for something that I can use for material components for other spells (specifically chromatic orb).
You can create rock or diamond, but not diamond from rock.
Part of the creation spell description says:
You can also use this spell to create mineral objects such as stone, crystal or metal.
So yes you can create stone and yes you can create diamond. However, there is no way for you (just with this spell) to make a diamond from a lump of stone no matter what tools you have. To have a diamond you have to start with diamond.
You cannot use any material from creation as a spell component
Using any material created by this spell as another spell's material component causes that spell to fail.
So, if you are intending to use the created material for any kind of spell component use, it will not work.
You can use an Arcane Focus to create a diamond without any pieces of diamond, then sell said diamond and purchase one that was not made with this spell.
PHB (Page 203):
Material (M) Casting some spells requires particular objects, specified in parentheses in the component entry. A character can use a component pouch or a spellcasting focus (found in “Equipment”) in place of the components specified for a spell. But if a cost is indicated for a component, a character must have that specific component before he or she can cast the spell. If a spell states that a material component is consumed by the spell, the caster must provide this component for each casting of the spell.