This can be a combination of cut and clarity. I think my diamond is very similar to yours (we have established that previously). Mine does this, too, in the head-on view. I have mine set in an open prong setting, so if I turn it to a slight angle away from head-on, it lights up again.
1. The sun only sub-tends light at about one-half a degree. This means, in the entire 180-degree hemisphere, the sun is one tiny point light source occupying 1/360th of the area a diamond is trying to "look" for light. So while you see the (very) bright sparkles such a small bright light source creates, confinement to only a half-degree limits what the diamond finds to give-back.
2. Human physiology. Your pupils get smaller in sunlight, further reducing brightness in the fore-and-background. The positive is that well-cut diamonds explode with more colored-sparkle... Why? Because when your pupils are smaller they are 'clipping' more dispersive fans so you're seeing more of them as color, rather than recombined white light.
In your picture the light from the sun is not entering the stone through the table, but glancing off the stone from the side. If you really must take a picture in the sun, put the sun at your back and hold your stone up so that the sun can enter through the front of the stone. You camera will probably still shade the stone too much, but it's better than how you're trying it now. One of the best ways to get a really good display, at least this time of year, is to stand under a leafless tree on a bright overcast day. The bright light from the sky and the dark reflection from the tree limbs makes a great contrast and also shows fire, (dispersion), nearly as well as being in a jewelry store.
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