Diamonds Are Forever by Eric Flint & Ryk E. Spoor (2004)
Nope, this isn't the James Bond novel, it's a fantasy novel that was included the Baen omnibus Mountain Magic. (The omnibus also included David Drake's 1991 collection Old Nathan and Henry Kuttner's delightful Hogben stories -- at least in print form: the electronic version dropped the Kuttner stories and substituted Manly Wade Wellman's 1998 collection John the Balladeer due to restrictions from the Kuttner estate. As you probably realize, Mountain Magic is an omnibus of Appalachian fantasy stories)
Clint Slade is the first person from his Kentucky family to graduate college. A geologist and computer scientist, Clint moved to New York where he met and fell for Jodi Goldman, an acoustical engineer. Jodi is a non-religious 6'2" amazon who often lapses into Yiddish phrases in her New York City accent. (NYC has ownership of a zillion accents; it's never specified which one Jodi has.) When the book opens, Clint is telephoning his mother to inform her that he and Jodi are engaged. Mama Slade (who is one not to be denied) insists that Clint bring his fiancee to meet the family.
So it's off to the woods of Kentucky. Turns out the Slades are well-to-do and live in a large rambling house filled with all the latest gadgets and gimmicks. It also turns out that the Slades are hiding a secret. Their compound is surrounded by an electrified fence...all the windows in the house are covered with steel shutters at night...every member in the family (including Clint) carry a crowbar with them.
The original Slade house was built in the early nineteenth century by Winston Slade, who happened to discover a network of caves on the property. While exploring the caves, Winston stumbles upon a network of pools, each with rounds pebbles at the bottom. Winston recognizes the pebbles as uncut diamonds. Scooping up a bunch of them, he is interrupted by...something. He escapes and the diamonds provide the foundation of his family's wealth. Over the years, Slades have gone back to the pool to replenish their wealth.
The creatures in the caves are kobolds, which (as a boy) Clint called nomes after the creatures in the Oz books. There was a time when they interacted with humans but that time is long past with only dim memories remaining. Whatever cataclysm happened to separate the races, it also separated the various tribes/clans of nomes with the different groups warring at each other. The nomes are a crystaline life form which are part of an intertwined life essence of the earth. The diamonds that the Slades had been purloining over the centuries are important in providing strength to the nomes; when cut and polished they lose their life essence.
Clint and Joni form a detente with the nomes, only to discover that an invading army of nomes are determined to destroy the local tribe and to set off a chain of earthquakes that would level a four-state area. Without the diamonds that the Slades have taken, the good nomes are defenseless. It's up to Clint and Joni to come up behind the invaders via a complicated series of tunnels from the Mammoth Cave.
The Slades are a typical family of "capable" people a la John W. Campbell, although they appear to lack a moral compass until Joni presses the issue. Joni is probably the most Campbellesque character in a story that would fit semi-comfortably in Campbell's old Unknown Worlds magazine.
That, I fear, is the problem with the novel. It reads as if it was designed to mimic an Unknown Worlds novel but the authors lack the skill (and Campbell's editorial touch) to pull it off. Efforts to provide humor are misguided and Joni's frequent lapses into Yiddish are just plain irritating in the most twee way possible. The effort to provide a semi-rational explanation of the fantasy element also misfires.
Diamonds Are Forever ends up being a mindless time-waster -- not something to be avoided at all costs, but more of a "well, there's nothing else to read tonight, so what the heck" type of book. The biggest shame is that it's packaged with much better stories by David Drake and/or Henry Kuttner/Manly Wade Wellman. They deserve better.