Gem and Gemology Image Gallery shows over a dozen varieties of gemstones in both raw and cut-and-polished form. Outline descriptions (clearly intended for a college course) and occasional videos round out the site.
Smithsonian Gem & Mineral Collection displays highlights of one of the Smithsonian Institution's remarkable holdings--even the Hope Diamond can be seen here.
Molecular Expressions maintains a dizzying array of exquisite photomicrographs. The materials examined are a diverse lot, including beer, buckyballs and silicon chips.
AT&T's Microscapes Gallery showcases an exhibit of museum-quality micrographs, made using a number of devices and techniques. The images mostly depict materials used in modern information technologies.
STM Image Gallery from IBM showcases the capability of the scanning tunneling microscope, a device that takes microscopy to the limit by revealing individual atoms. The images are neatly arranged in a virtual gallery space.
Small is Beautiful, contrary to its name, is not much to look at. Rather, it consists of a definitive set of links to online nanotechnology resources (many of which do contain stunning images). When a better molecular machine is invented, you will be able to find out about it here.