Board Game Arena

What possible motivation could there be for spending a gold on a card that you can afford without using it? If I am sitting on let's say 3 emerald and I know my opponent needs emerald for a card they reserved etc, I may want to spend my gold to keep them locked out of emerald. Just another example ...

I might just be missing this, but how do I choose to use gold instead of a gem? As an example: I had 2 diamonds and a gold, but I could not figure out how to use 1 diamond and 1 gold to buy the card, the game effectively forced me to use my diamonds first. So, am I just missing how to select what yo...