Vintage 8K solid gold coin miniature Maximiliano Emperador Imperio Mexicano 1865. 8k gold could tarnish from sulfur and ammonia in the atmosphere. Copper, in particular, likes to react with oxygen (moisture) and this can lead to discoloration or tarnishing greenish to brownish. This is normal process. The coin is over 35 years old. The measures of the Gold COIN 8K: 1,1 cm diameter. Very special and rare to find.