Based on the Random House Unabridged Dictionary, © Random House, Inc. 2021, Collins English Dictionary - Complete & Unabridged 2012 Digital Edition Trigonal (rhombohedral) crystals of quartz. Get a Britannica Premium subscription and gain access to exclusive content. Since 1960 the Commission on New Minerals and Mineral Names of the International Mineralogical Association has reviewed descriptions of new minerals and proposals for new mineral names and has attempted to remove inconsistencies. Since a mineral has a definite composition, it can be expressed by a specific chemical formula. Because of this feature, minerals are classified as crystalline solids. This allows you to deliver the oil directly into your rectum. Do you know what they mean? Solids that exhibit no such ordered internal arrangement are termed amorphous. Be on the lookout for your Britannica newsletter to get trusted stories delivered right to your inbox. 1375–1425; late Middle English