New to Crystals, help me decide please

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I've always been fascinated with crystals, lately I've been researching them to see how they would help me with Astral Projection, and life in general. But I am not sure which crystal is for me.
What I want to do is to just get 1 really good and beautiful crystal and make it into a pendant to carry around with me all the time.

I have a couple questions.

1: I heard you can only program ONE intent to a crystal, are there any crystals out there that can handle more than one intent?

2: Can you change a crystals intent? Like at night, it can help me Astral Project, but at day it can just protect me or help me keep my mind at a higher awareness?

3: If I buy a crystal and have it cut, so it looks nice and is changed to a nice shape, will that affect it in any way? I just don't want to keep it messy looking and I want to make it look better so I can put it as a pendant to keep with me.

4: I can't choose between Amethyst, Quartz, and Ametrine
Honestly, Ametrine looks incredible. But I am not sure which is best for my purposes and which would be strongest.




Note: I just plan on making them into a pendant that looks almost exactly like this one: https://www.healingcrystals.com/images/amethyst_pendant.jpg

^Is that sized crystal good enough? Or does it have to be bigger to have an effect on me/do anything?