Its not just one thing or one course or one techinique .... its lots of little things. While the first thing that might come to mind when you think of a good designer is "drawing skills", I don’t think this is true. A good designer solves problems and communicates those solutions in a creative manner. How they communicate is up to them.

So with no set expectations in regards to drawing I believe a good jewellery designer .....

Has willingness to listen to what the client actually wants and needs - read between the lines and ask questions. get to know your client so you can provide suitable solutions

Can think creatively to solve problems - flexibility sometimes leads to better solutions. Always be open to inspiration and ideas and always have a back up plan that you can pivot to if your first idea doesn't work.
Understanding concept design - an openness to see the beautiful details in absolutely everything around you and to find inspiration in those details. (Check out my concept design prompt sheet here if you are looking for some inspiration)
Having an understanding of the design principals and how to use them to your advantage. (Find a copy of my favourite design elements and principals here)
Presentation and story telling - in any format but find a format that suits your style.
Has the courage to communicate their ideas - getting your ideas out is always the first step but understanding that they may come with criticism
Has an understanding of manufacturing and materials - knowing how something is made and how it will behave is a massive requirement when it comes to providing the best solution. Understanding the steps before and after you are important to creating a lasting design.
A willingness to have fun and not take themselves to seriously.

Its not always about drawing a picture, drawing is just one tool of a good designer.