Among the precious stones, the most extensive foreign trade during the Gupta age was in sapphires. Sapphires are a type of precious stone that are typically blue in color, although they can also be found in other colors.

Option 1, Diamonds: Diamonds are also a precious stone, but during the Gupta age, sapphires had a more extensive foreign trade.

Option 2, Ruby: Rubies are another type of precious stone, but they were not as extensively traded as sapphires during the Gupta age.

Option 3, Pearl: Pearls are not classified as precious stones, as they are formed within living organisms and are not mined like diamonds, rubies, and sapphires. Therefore, they would not have been part of the extensive foreign trade of precious stones during the Gupta age.

Option 4, Sapphire: This is the correct answer. Sapphires were the most extensively traded precious stone during the Gupta age.

In conclusion, the correct answer is option 4, Sapphire.