The use of the particle "de" Is there anybody who can help me, please?..

I have studied that in this setence: "Zhongguo de youpiao"  zhongguo characterizes youpiao e that's why we use the particle "de", but I found this sentence: "Zhongguo cha" where the word Zhongguo seems to characterize the word "cha", so why don't we use the particle "de" in this case? 

非常感谢你的帮助!
It's the same case as Engligh right? We can also say in English: China Tea, or Chinese Tea. As long as omitting "de" does not introduce ambiguity, it's fine. However, it will sounds more like a proper noun instead of adj.+noun.