This is a great article posted on another site (who reposted it from another site, who took it from another), but I thought many would enjoy it here. It is The Lazy Man’s Guide for Constructing a Call of the Cthulhu Adventure, written by Sandy Petersen, original author of the Call of Cthulhu.
Also called "pale-yellow witch" by alchemists, this mineral is known to possess a peculiar attribute. When found, a Yupiorite will appear the palest yellow. Rather than crystalline in structure, Yupiorite occurs in weird, smooth, ovaline shapes, as if already carved by skilled hands to serve as ring or necklace ornaments. Yupiorite somehow detects and reacts to mood. When the wearer of the gem is content, calm, and happy, the stone will remain the palest yellow. As the person gets more excited, angry, or otherwise stimulated, the mineral will darken progressively to a dark corn-yellow in color. Why the gem reacts this way to sentient mood swings, is still debated by gemologists and alchemists alike.
It is said that the Elven Halls of Vala-Aluduwy are resplendent with wall-sized mirrors of pure Yupiorite, showing plainly and ironically, the emotions of everyone present, despite the Elven love of restraint and stoicism.
"Cave-grass" or "cave-pine" is a deep forest green in color, rare and often mistaken for other minerals, though otherwise mundane. Crystals form into tiny, ultra-thin, needle-like clusters by the hundreds of thousands, creating vast dark green bursts and structures, resembling evergreen conifers, if viewed by any sort of light. Despite its ephemeral shape, Aragdulose is only second to a diamond in hardness.
Dwarves are said to keep these mineral "trees" in their homes, putting them up during festive family holidays, leaving presents beneath them, for kin to open.