During the quest "Dulled Steel, Cold Forge" in Dragon's Dogma 2, you'll be sent to Digger's Ruin to mine Glimmercoal. Once inside the cave, you'll discover that there's also a similar resource called Glymercole, but is it useful?
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Glimmercoal and Glymercole are both crystal minerals that you can find in Digger's Ruin near Bakbattahl, but Glimmercoal (the one you need to repair the Dulled Regalia Sword) shines with a green glow in the dark, while the other does not.
Glymercole appears to be an inactive version of Glimmercoal, and thus isn’t all that valuable. It’s placed in a cave as bait to line your pockets and make it difficult for you to find the one you want – since it’s useless to the Arisen outside of its selling price. But at 150 coins a pop, if you can afford the extra weight in your backpack, you should grab it.
The best way to tell Glimmercoal from Glymercole is to turn off your lantern, as the former will start glowing. If you shine light on it, the crystal will go out, but after a few seconds of pure darkness, it will start shining again. You can also approach the shards and the UI tooltip will differ depending on what material it is. While the simple Glymercole says "Explore", Glimmercoal says "Collect".