My first recommendation is to use a blue stone since the color blue can represent sadness.“Too much blue can create feelings of melancholy, negativity, sadness, self-righteousness, and self-centeredness. Too little blue brings about qualities of suspicion, depression, stubbornness, timidity, and unreliability.”http://www.bourncreative.com/meaning-of-the-color-blue/There’s a ‘Blue’ gemstone list in this post.In addition, here are some links to gemstones that help with sadness and depression:https://crystal-cure.com/article-grieving-stones.htmlhttps://www.crystalvaults.com/crystal-reference-guide/crystals-for-depressionhttp://www.beliefnet.com/wellness/health/three-crystals-to-help-you-fight-depression.aspx