Gems, jewelry and art HELP

Just wondering how the players are supposed to know the value of gems and art?  The DM often uses gems as treasure, and it seems that there is no way of determining even an approximate price - and giving an approximate price seems wierd for characters not knowing anything gems, jewelry or money in general.  Thanks:) 

You would probably be better suited to asking this on reddit as this is not a Roll20 specific question and more of a general D&D 5e (?) question. However, it depends on your game style. Some DMs just tell the players right away how much these items are worth because they don't want to have to remember it themselves. Others make the characters roll for it, you can base it off a character's Intelligence check or maybe even add in a tool proficiency. For example proficiency with a jewelers kit would allow the character to better approximate it's value (I would even rule that if the character had proficiency with the jeweler's kit as well as actually had the jeweler's kit in their possession that they would automatically know the value of gems). Art objects for me would be a different proficiency depending on what the art object is. Maybe for paintings I would require proficiency in the painter's supplies.

From the Roll20 Community Code of Conduct : The Roll20 Forums exist to discuss topics directly related to the use of the Roll20 program. Anything that more fittingly could be discussed on another website SHOULD be discussed there. Here are some good places to discuss this topic: /r/rpg /r/dnd /r/dndnext /r/gamemasters