Best to go with a medium/large fighter ship, I used an Asp with military drive (less cargo space required), shields and beam lasers etc, the Cemiess system is in anarchy so its highly likely you'll get attacked by pirates (normally around 2-4 times, sometimes 3 at once) and these guys don't muck around...
...once you get there, search the BBS for the contact in the previous post, sometimes he won't be there so just advance time until he is. Don't use any other contact as it will be a Police trap.
Sell everything from you ship, leaving yourself just the star drive, fuel and autopilot, you won't need anything else for the return journey, giving you the maximum cargo space to fill up with Gemstones, you'll get a bit of money by selling your equipment, but each cargo space is worth much more money empty
You won't even need Atmospheric Shielding
Once you are full to the brim, set your destination system and launch hitting hyperspace as quickly as possible, 3 more jumps you'll be in Barnard's Star system, where you never get attacked and Radioactives (by-product of Military fuel) are not illegal (as they are in Sol).
Sell your Gemstones, re-buy your equipment, rinse repeat!
There are places where you get BBS buyers paying a premium for Gemstones, but they are a bit far away, you'll definitely need a well armed ship (loosing cargo space that could be for Gemstones).
It is possible to do this in a trader*... but it costs alot to arm it, harder to defend, of course you'll need to make more jumps and when you finished doing the trade runs you'll have staff to fire
*to me it didn't feel, uhmm... right! I preferred to think of it as a quick dash, daring, risky mission... rather than a legit trade run.