Platinum 
Gold= 10 make 1 platinum 
Silver 10 make 1 gold 
Copper 10 make 1 silver 
Gems and other minerals will be assessed and traded with in individual situations, and will be up to the player receiving the gem or mineral how much he/she is willing to assess its value at. 
Some costs and breakdowns for ideas on paying for things: 
A room at the Inn: 1 Silver per week for a simple room with 1 meal, 2 silver per week for a room with accessories and a meal per day 
A meal: usually 1 copper for simple stew and bread and water, 2 coppers for a more full meal, with wine etc. 
A basic longsword: 5 gold 
Shield: 3 gold 
Dresses and tunics (basic): 1 or 2 silver and increase as they get more fancy.