Ask yourself: why is there this herb or this crystal in the spell in the first place?
The person who originally came up with the spell didn’t put it there at random, they did so because they had a specific plan in mind – a specific reason. You work out what that reason was, and you may realise – there are other methods of achieving the same goal not necessarily using the original herbs or crystals, or indeed any at all.
So yes, if you can breakdown how and why spells are constructed you could do witchcraft without either herbs or crystals, even to the extent of learning to create your own spells. You might even realise that you know more than the person who originally wrote the spell!
Incidentally, I can offer my opinion about herbs and crystals which may help you. The main reason they are used in magic is because they have Astrological properties which coincide with the purpose of the spell. There is more than one way of invoking Astrological energies, this is the sort of thing which can be researched.
Or of course, you may get to the point where although you technically can work without herbs or crystals, you can make an executive decision that working with a herb or crystal is most convenient for you in a particular instance.