Crystals are the game's premium currency. You can spend them to buy Chromium Data Cards to unlock more characters. They also can be used to get more energy, reset the time counters, etc. Log in daily for a month to collect the daily reward to save crystals. Complete the daily missions to earn crystals. Complete campaign under the hard difficulty setting. The normal difficulty setting has crystals as a reward upon first time completion. Earn achievements to earn crystals after completing some of them. Get a high enough rank in the Squad Arena to earn a daily reward of crystals.