"But a small percentage is still a substantial amount of money," she added.
The trustees' decision carries more symbolic than financial weight, but it is a major victory for a rapidly growing student-led divestment movement that is now active at roughly 300 universities.
At least 11 small universities have elected to remove fossil-fuel stocks from their endowments, but none approaches Stanford's prestige or national influence. Tuesday's decision seems likely to increase the pressure on other major universities to follow suit.
Among other universities, Harvard has resisted student pressure for divestment, and one student was arrested on Thursday after pro-divestment activists blockaded the entrance to the school's administrative offices. Stanford's trustees acted after Fossil Free Stanford, the campus branch of the movement, petitioned the board to re-evaluate the university's holdings in energy companies, Ms. Lapin said.
Yari Greaney, 20, a Fossil Free Stanford organizer, said the group was "very proud of Stanford taking this leadership position." Nationally, leaders of the divestment movement praised the school for its decision.
As a global institution, Stanford "knows the havoc that climate change creates around our planet," Bill McKibben, the president and co-founder of the environmental group 350.org, said in a statement. "Other forward-looking and internationally minded institutions will follow, I'm sure."
Maura Cowley, the executive director of Energy Action Coalition, an assemblage of groups active on climate change issues, called the decision "a huge, huge victory."
"Their decision, coming from such a major university and from such a huge endowment, shows that the coal industry and other fossil fuel industries are quickly becoming relics of the past," she said in an interview.
The trustees began studying divestment after Fossil Free Stanford petitioned them to re-evaluate their holdings of energy companies. An advisory panel that included students, faculty, staff and alumni spent roughly five months studying the issue before recommending that coal stocks be sold, Deborah DeCotis, the chairwoman of the board's special committee on investment responsibility, said in an interview.
Among other deciding factors, Ms. DeCotis said, the panel noted that coal produces the most carbon per British thermal unit of any widely used fossil fuel, that practical alternatives to burning coal are available, and that the university was not dependent on coal or coal-derived products.
Other fossil fuels did not meet some of those criteria, but "this is not the ending point. It's a process," she said. "We're a research institute, and as the technology develops to make other forms of alternative energy sources available, we will continue to review and make decisions about things we should not be invested in. Don't interpret this as a pass on other things."
Ms. Lapin said the school is already asking its investment advisers to review endowment holdings and sell stocks of coal companies. The order covers mutual funds with coal stocks as well as investments in individual companies, she said. ---------------------------------- SIDEBAR PHOTO: The campus of Stanford University in Palo Alto, Calif. The decision by trustees to get rid of stock in coal-mining companies was a victory for a rapidly growing student-led divestment movement. Credit Thor Swift for The New York Times ------------------------------------ A version of this article appears in print on May 7, 2014, on page A15 of the New York edition with the headline: Stanford to Purge $18 Billion Endowment of Coal Stock. ***************************************************