Minerals. They come in many different colors, and stronger ones scratch weaker ones. Graphite is used to write with (communicate) and diamonds are considered luxurious. Grouped together, minerals make rocks. Coal can cause black lung (disease) and while being mined it can cave in and crush someone (sudden death). More than 3000 minerals are in existence, (hundreds of names), but I am categorized under minerals.
Today's brain teaser courtesy of Braingle.com.