Check the map of the museum properly. You have to reach the Emerald chamber and steal the Emerald.
You will have to collect some keys to open some closed doors. Try to understand the color of the keys and doors.
The precious Emerald is ofcourse Green. Steal the emerald and escape from the building before the cops arrive.

Use Arrow Keys to move. Stay alert from the laser beams and cameras to avoid being caught.
Hint : Check the map to discover which room you are in if you feel lost. Pay attention to the instructions on the bottom left.