Hello.
New artist, working on [trying to] understand shading and color ramps, as well as general pixel fundamentals. I think studying gems is a worthwhile endeavor, as they have extreme shifts in color and hue [and at such speed!]. This topaz was a fun challenge.
I used this image as a reference when working on this piece.
i.etsystatic.com/20574148/r/il/b3f249/3151638206/il_fullxfull.3151638206_tihw.jpg
Thank you for viewing my submission.
- F. | 
I think you did a pretty good job on it! I think to take it to the next level, next time, you could try using some different colors for some of the more stark parts or making the image a little bigger as the dark spots stand out a bit too much on the bright yellow.