My mom used to make all the spice powders at home. Every time I visited her, I would bring all the spice powders like garam masala powder, rasam powder, sambhar powder, chili powder etc. back home with me. For the past 10 years, I have stopped that habit and learnt everything from my mom.
I used to buy readymade powders in the market, but I read everywhere that there are chemicals and preservatives mixed in them to make them last longer and look more attractive. Since I heard that I have stopped buying even turmeric powder in the market and hand grind all of them. They can be easily made and will last a long time at the right temperature. Also, the taste you get from homemade powders will be completely different and make your dishes even tastier.
Heat a pan. Add coriander seeds (1 cup) and dry roast on medium flame.
While they roast, you will start smelling the oils release from the seeds and a nice aroma will waft through.
When the seeds turn a little dark, take it off the heat and let it cool well.
Dry grind it to a fine powder.
Store in an air tight container and it will last for a minimum of 3-4 months.