Countable and Uncountable Nouns
Countable nouns are nouns that can be counted, while uncountable nouns are nouns that cannot be counted. Countable nouns can be either singular or plural and can be used with numbers and determiners such as "a," "an," and "many." Uncountable nouns, on the other hand, do not have a plural form and cannot be used with numbers or determiners that indicate quantity.

Identifying the Countable Noun
In the given sentence, "Good friends are beauty of life," the word "friends" is a countable noun. This can be determined by considering the following factors:

1. Plural Form: Countable nouns have a plural form that can be used to indicate more than one. In this case, the word "friends" is the plural form of the countable noun "friend."

2. Use with Numbers: Countable nouns can be used with numbers to indicate quantity. For example, you can say "three friends" or "many friends." In the given sentence, the word "friends" can be used with numbers to indicate the number of friends.

3. Use with Determiners: Countable nouns can be used with determiners such as "a" or "an" to indicate a single item or with determiners such as "many" or "some" to indicate multiple items. In the given sentence, the word "friends" can be used with the determiner "good" to indicate multiple good friends.

Therefore, based on these characteristics, it can be concluded that the countable noun in the given sentence is "friends."

Conclusion
In the sentence "Good friends are beauty of life," the countable noun is "friends." Countable nouns have a plural form, can be used with numbers, and can be used with determiners to indicate quantity. The word "friends" in the sentence meets these criteria, making it a countable noun.