I list them because they're smart, relevant, live, legal, solid, discriminating, and original. Smart sites know what you're looking for and get it for you with a minimum of clicking and waiting. This is my first rule, because the Web will be slow for a long time to come, and a site had better be extra special for me to ask you to wait for it. The Southern California Earthquake Center is a smart site. I won't show you any dumb ones, although there are some GeoCities sites that I list despite their well-known annoyances.
Relevant sites focus on Earth science in some aspect.
I leave them alone unless they have a good section on mineralogy. Live sites stay fresh, and I don't mean blinking text and whirling icons. Links don't go bad, tired content is brought up to date, design flaws don't fester for years. Some widely linked sites fail this test. The Earth Science Site of the Week site, for example, was a great idea, but it's been dead for two years, and it hasn't had a site every week since May 1995.