You have to focus the magnifying glass (which is in the bottom right corner) on the diamonds, and find the strange inscription on one. It is the second one in from the lower left.
It's at the end of the rail track things in the Diamond Mines in the pile of gems.
At the end of the Diamond Mine, you have to pick the White Nabooti Jewel from a pile of diamonds. Use the small loupe (magnifier) at the bottom left to look at all of the diamonds. Only one, near the lower left, has the triangular Nabooti symbol on it.
At the end of the Diamond Mine, you find the white jewel among the diamonds by using the small magnifier at the bottom right. The jewel has a triangular symbol on it. When you see it (third jewel from bottom left), you can pick it up.
The white jewel is at the end of the Diamond Mine, mixed in with a pile of diamonds. To find the right one, use the magnifying glass at the bottom right. It should be just in from the bottom left of the pile. (see related question)
At the end of the mine, you choose the White Jewel from the pile of diamonds. Use the small magnifying loupe at the lower right, and you will find that one diamond (the Nabooti jewel) has a triangular mark or inscription on it. You can then pick it up. (for the full solution, see the related question)
At the end of the Diamond Mine is a pile of diamonds, one of which is the White Jewel. Look through the diamonds with the magnifier provided (at bottom right). The White jewel should be the third one up from the bottom left corner. (It has a triangular symbol on it.) When you have examined it under the magnifier, you will see a prompt to "take" the jewel.
The White Jewel is among a pile of diamonds at the end of the Diamond Mine. You will need the hard hat from the Safari to enter the mine. (see related questions)
It is a triangular mark, visible using the magnifier at the bottom right of the window. It should be the third diamond from the bottom left in the pile.
The White Jewel of Nabooti is on the Diamond Mine. Once you have located the pile of diamonds, use the magnifier loupe (bottom right side) to examine the diamonds. The White Jewel has a triangular inscription on it. Pick it up. (It should be the third diamond from the bottom left side.) (see related question)
You cannot just click on all of the diamonds in the pile at the end of the mine cart ride. You must use the jeweler's loupe (magnifier) at the lower right, and examine each of the jewels until you find the one with the triangular inscription on it (the Nabooti symbol). This is the White Jewel, and you can click to pick it up. (It is one of the three on the lower left of the pile.)
The white jewel is in the pile of diamonds in the diamond mine. Use the magnifier at bottom right to examine them until you locate it. Once you have all FIVE jewels, you can return them to the Nabooti totem pole in the museum. The placement order is purple, green, red, white, and blue. (see related question)
In the pile of diamonds at the end of the Diamond Mine, the White Jewel is the only one with a special triangular mark on it. Pick up the magnifying loupe at the bottom right and look at all the diamonds. The white jewel should be the third one from bottom left.