I think giving players gems instead of gold is a very effective game mechanic.  Gems are lighter so they don't weigh down a PC.  They usually have to be converted into gold or other coin before being spent. 

As a story weaving element, I try and give the PCs gems without stating what type of gem it is and it's worth.  I say, "There are five red gems, pretty clear about the size of the fingernail on your smallest finger, with a faceted cut roughly in the shape of a rectangle."  Then I give the players a photocopy of a drawing of the gems and let them convert the gems into gold at a town. 

The party could try to use them to pay for armor and weapons at an armorer without knowing what they are worth, or they could seek out a gem cutter and ask for an appraisal.  Even with the appraised gem, if they want to convert it to gold, they have to find someone that wants to buy the gem for that much.  Along the way, they could talk to a merchant with a good scale that might weigh the gems so they could find out the gems weighed 250 grains of sand or some such value, and they could write that on the drawing next to the gem.  Verifying the red gem is a ruby and not a garnet requires someone with deeper knowledge.

It can create opportunities to RP and explore the town.  What happens when a party of rough looking characters, clearly in need of a bath and a night's sleep, come into a bar and order drinks, and then they ask where they might find a jeweler able to appraise gems?

As a DM you could make a chart of gem value based on type, size, cut and clarity and keep it in your notes.  You just need to give the players a number when they get a gem that you can cross reference in your notes when they want an appraisal.

This is too much detail for some groups but in my circle of friends this creates interest.  You just have to make sure there is an adequate supply of jewelers and gem cutters to help the party.

That’s genius. I have always liked the idea of handing out gems but never really understood why. The obvious answer is that it’s another hook. Also my group tends to not play with encumbrance, so it would be pretty easy to tell a character that, “sure, you have 15,000 gp on you, but a large percentage of that is in gems, and if you want to cash them in you will have to go to the Night Market and find Veldor Sprektiousmagrrix, the head of Svirfniblum Surface Trading Expedition ...” 

Handouts with pictures of gems would be cool too, but maybe a little labor intensive if they aren’t magic or really important t the plot?

Gems are also spell components.

And tasty to certain creatures, Xorns, Gold Dragons, so on

maybe you could make a magic bag that converts whatever is put into it into coins.

I hope this comes through.

https://www.bing.com/search?q=gem shapes&form=EDNTHT&mkt=en-us&httpsmsn=1&msnews=1&plvar=0&refig=81f1aef0810f433d8230b0ed3345ea51&PC=ASTS&sp=-1&pq=gem shapes&sc=6-10&qs=n&sk=&cvid=81f1aef0810f433d8230b0ed3345ea51

I would make handouts by copying this diagram and then print two copies.  One copy would go into my DM notes.  The second copy would be cut up into twenty five individual pictures and each taped to a piece of paper so I could photo copy them onto index card sized handouts.  I would then have a few stacks of these index cards where I could add a not about the size of the gem and the color.  I could give the index card to the player and they could try and estimate the value themselves or they could take it to a gemologist (gem cutter, jeweler) and have him provide an estimate of the value.

As a player, I can say that I definitely get a lot more excited about finding gems, and art pieces, and other random objects in a treasure pile than I do about coins.