Answer: Viridian Emerald Pharaoh
Why:
The key terms are the jewel names Alexandrite, Emerald and Sapphire. There's a thing called change color when gemstones are exposed to light or heat. The jewels in the case sound like they were swapped when they actually changed color. The Viridian Emerald has the same color so it won't change. The Green Alexandrite shines green under daylight and strong light spectrum. However, since the museum lights are out it turns red when exposed to incandescent heat such as candles. The same with the Pink Sapphire, but it changed from pink to green. The pattern the jewels went in from left to right was the Green Alexandrite Emperor, Pink Sapphire Queen and Viridian Emerald Pharaoh. Since the robber took the one on the far right; he took the Viridian Emerald Pharaoh.