The one with another dream collection

My brother started collecting coins when he was younger.  He got it from my dad.  I, on the other hand, doesn't know anything about antique coins. Why is it valuable to some collectors? I had a conversation once with a fellow friend who is quite engaged in coin collecting and would unquestionably research information about a coin that can complete his collections. Fanatic I can say to that friend of mine. He did mention about Mexican silver. He said that many of it isn’t exactly sterling silver so one should do acid test to ensure its purity. Oh well, I wasn’t fascinated much about the conversation but I did learn a thing or two about buying silver. Thanks bro!