Gemstones are one of the most important resources in the game for many reasons. They can be sold for lots of money and they are also a key crafting ingredient for the high end items. Royal steel/cloth require lots of gems to produce. IF you want to progress quickly to get your droid, you will need to gather lots of gemstones.
Gemstones in Forager include Amethysts, Rubys, Emeralds and Topaz. All of these will drop normally from ore, but if you know what you are doing, you can unlock them without needing to do any work at all. This is more of a gem farm as you can set it up and go make a sandwich. Come back and you will have earned loads of gems and also a bunch of easy XP.
Unlock the Prospecting Skill
This skill is three places over from Foraging on the skill tree. It will increase the cance of rare gems dropping by 10%. You also get a better chance of finding something while you are digging, but thats not important here.
Unlock the Deposit Skill
This skill is two places down from Prospecting. This one will unlock quarries. Once you build a quarry, ore will randomy start appearing around it in very large amounts. Every day you will get lots of random ore appear. This will make it a lot less work for you to mine gem stones.
Unlock The Automation Skill and Build LOTS of Mining Rods + Quarrys
This skill unlocks mining rods, which are the holy grail of this game. Getting gem stones easily becomes even easier with this as it does all the work for you. Place a mining rod in the middle of an area of land and it will destroy all of the ore veins for you. Gems willl continue to drop.
Sit back and wait
This is the easy part. The mining rods will work away on their own. The more of them you have with more quarrys built nearby the faster this works. Find a safe place to stand and go and do something else. The mining rods will destroy all of the ore for you and even collect it. Once you come back after an hour you should have a mountain of Gem stones.
To make sure you do not hit trouble. Create a vault and store the stones in here. At least you will be guaranteed that all the gems that dropped will get picked up and stored in your inventory.