What are the Gems for in Idle Supermarket Tycoon?

Gems in the game are the premium currency. There are several ways to earn them for free but most notably through VIP customers (equivalent to watching ads), and through beating the missions. Gems are basically used to purchase passive skills (and upgrade them as well) that helps you in improving the productivity of your employees, the volume of traffic in your supermarket (even customer’s strolling pace), and a lot of things that has something to do with improving your profits. Below are the “Epic Upgrades” which you can unlock with gems.

notes: ( ** ) - increases value per upgrade

Seal of Quality - will increase profits by 2 **

Compulsive Customers - customers move 5% ** faster

Produce Manager - will increase produce department’s productivity by 50%

Bakery Manager - will increase bakery’s productivity by 50%

Butcher Shop Manager - will increase butcher shop productivity by 50%

Seafood Dept. Manager - will increase seafood dept. productivity by 50%

Clothing Dept. Manager - will increase clothing dept. productivity by 50%

Perfume Dept. Manager - will increase perfume dept. productivity by 50%

Electronics Dept. Manager - will increase electronics dept. productivity by 50%

Telecom Dept. Manager - will increase telecom dept. productivity by 50%

Jewelry Dept. Manager - will increase jewelry dept. productivity by 50%

Perfume Dept. Manager - will increase perfume dept. productivity by 50%

Car Dealership Manager - will increase car dealership productivity by 50%

Real Estate Agency Manager - will increase real estate agency productivity by 50%

Express Delivery - increases the delivery vans' profits by 50% **

Robotic Assistance - cashiers attend to customers 5% ** faster

Family Marketing - will increase the chance of multiple people traveling in one car by 5% **

Labor Subsidy - will reduce salary cost by 2% **

Provider Agreement - will reduce cashier cost by 2% **

VIP Marketing - earn greater rewards from VIP customers by a 10% ** percentage **

Tax Cuts - will increase city earnings multiplier by 1% **

Rush Hour - will increase the maximum time for advertising campaigns by 30 minutes **

Different R&Ds - reduces a pertaining department’s (of your choice) improvement costs by 2% **

Quick Tip:

Just play through the game without paying much attention on the missions on the first few cities (probably until you get to Hongkong at least) and then pick the most important "Epic Upgrades" first. Among the passive skills, what you'd want to focus upgrading or unlocking are the following:

Compulsive Customers - because faster customers will equate to faster flow of income in your supermarket

Produce and Bakery Manager - because it will help boost you whenever you restart from one city to another by giving you better flow of funds every single time you re-build your supermarket from scratch.

Car Dealership and Real Estate Agency Manager - because they generally are the best income generator in your supermarket. It will help a lot to boost their productivity by 50%.

VIP Marketing - improves the value of gems you receive from VIP customers.

Free gems in the game can easily be farmed by simply taking on the missions. Missions usually reward you with gems upon successfully meeting its requirements and it does give you a hefty chunk of it to boot. For instance, by the time you get to New York city, you'll probably have saved around 2-3000+ gems just by playing through the missions.