My recommendation is to narrow down what you want it to look like with reference photos.

Example - this one has a holder for the gem. You could do this with small chunks of larger PVC piping fit on each other, 3d printed,

https://www.amazon.com/King-Scepter-Prop-Apparel-Accessories/dp/B08CS1WXFL

This one has a gem held into place with faux wood looking clay or glue.

https://www.amazon.com/Disney-Maleficent-Movie-Costume-Staff/dp/B00IKCMPPI/

This one is just more of a cage.

https://www.ebay.com/itm/314285000079

Another with a dragon claw holding the orb.

https://www.amazon.com.au/Scepter-Halloween-Decorations-Costumes-Accessory/dp/B08DR85MXT

The theory being gem to the topper. The topper slides over the end of the dowel. Screw through the topper into the dowel.

You can buy fake gems from etsy, ebay, and amazon of varying sizes or cast your own. Additionally you could repaint a christmas tree ornament like a gem.

I also have some sandpaper to hopefully make a spiral shape down the dowel to make it look more magical and some glitter for shininess

Two alternatives - use tape to spiral down the dowel - leaving a gap - and paint the exposed areas with what ever color and glitter. Carefully remove the tape and you should have clean lines. If you reverse the direction afterwards you can add a reverse spiral.

Second - use glitter hot glue to make your spiral down the dowel.

https://www.amazon.com/Glitter-Glue-Sticks-Hot-Gun/dp/B0044SD0PG