The Kos of Kiddush represent Shabbos in its beauty and splendor. It is what the man of the house holds up and proclaims Shabbos. Throughout the generations every family had a treasure Kos, but what was it made out of?
Most people were, and are, Makpid to use a silver Kos. Why don't we use gold which is a more precious metal? The Kaf HaChaim brings that the gematria of Kos is 86 which is the same as the gematria of Elokim. Since Elokim is the SHem Hashem of Din we prefer silver which represents Chesed to sweeten the Din. It is also known the wine represents Din so we put our Din in a vessel of Chesed to soften it. Gold on the other hand represents Din so we avoid using it for a Kos. It is known however that the Divrei Chaim of Sanz and the Divrei Yoel of Satmar both used a Kos of gold.
Others like Rav Naftoli of Rupshitz and the Kotzker Rebbe and Chasidus that temmed from him, use glass Kosos. Rav Yaakov Emden says that although his father the Chacham Tzvi had no shortage of gold and silver goblets, still he used a glass Kos. The reason given for using glass is because one should see the appearance of the wine. Only through a glass goblet can we fully appreciate the beauty of the wine. (See Heichal HaShabbos 2:31)