It's the third jewel from the bottom left. It has a swirly marking on it, but you have to use the magnifying glass to see 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 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.
It's at the end of the rail track things in the Diamond Mines in the pile of gems.
The white jewel is in a pile of diamonds at the end of the cart ride in the Diamond Mine. Use the magnifier at the bottom right to examine the jewels, and one will have a triangular inscription on it. This is the White Jewel, and should be the third one from bottom left. When you see the mark, you can pick up the jewel. (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.
At the end of the Diamond Mine, the white jewel is in a pile of diamonds. Use the small magnifier (loupe) at bottom right to examine the diamonds, and you will see one that has an unusual triangular inscription. This is the white jewel, and you can take it. (It is the second one from bottom left.)
The White Jewel is the only one of the diamonds with a triangular mark on it. Use the magnifier (bottom right) to look at all of the jewels. It should be on the lower left side of the pile.
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 diamond mines mine, look for a white jewel with a special mark on it
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)
You have to go through the entire mine and at the end you will find a pile of loose diamonds. Use the magnifier at the bottom to look at each one, and you will see that one of them has a triangular mark. This is the White jewel, and you can pick it up. (see the related solution question)