Do you want to look a bit more formal or professional than your audience?
My thoughts - I recommend no bracelets because they clink and bang on the podium, ditto a long or swingy necklace. But sparkly or bright color earrings (or glasses!) draw attention to your face, which is good. If you loke scarves, they can bring a shot of color near your face too. I always wore a solid but strong color (or two) when speaking. If you have a podium (likely), the bottom half of you won't be visible. Small prints can read as muddy or blurry from a distance. Black looks flat without some touch of color.
Think about whether you will be seated for any reason. Or walking around a stage with a lapel microphone. Flowy materials are appealing if you are moving.
Don't choose an uncomfortable outfit, but something a bit more dressy or formal can sharpen your attitude and delivery as well as your look.
And the best advice I ever got - remember to look both left and right when you speak. We tend to aim our gaze in one direction according to our handedness. So I always looked to the right.
And if you are passionate about your subject, that is EVERYTHING.