GIA (Gemological Institute of America) on Instagram: "Obsidian is a volcanic silica glass that forms when lava cools so fast that crystals don’t have time to grow. There are several beautiful varieties of obsidian, including snowflake obsidian and rainbow obsidian (seen here). Rainbow obsidian is unusual in that it contains layers of inclusions that are oriented parallel to the lava’s original flow structure. When light reflects off these layers, it is broken into spectral hues creating iridesce