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The DMS Team

Macella O'Neill

Macella O'Neill and her partner Charles White started Diamond Mountain Stables (DMS) in 1983, returning to their hometown of Calistoga after they had both gone to college. Macella graduated from UC San Diego with a double major in history and biology and was poised to continue her education, but decided to take a "short break" and indulge her love of horses and start DMS w/Charlie and his love of business. The rest, as they say, is history!  

It's been quite a journey from local 4-H leader to World Cup competitor. Along the way Macella has coached year end winners in every division: ponies, equitation, hunters, jumpers and won medal finals as well as put students on the highly competitive Zone 10 Young Riders teams. Enormously popular as a clinician, Macella has taught clinics all over the world as well as judging when time allows.  

In an effort to participate in the industry that has given her so much, Macella has been the president of the NorCal Hunter Jumper Association for several years and was the NorCal liaison to the Pacific Coast Horse Association and an AHSA rep as well. For fifteen years Macella, and the entire gang at DMS (really Charlie!) ran the enormously successful Napa Valley Classic horse show which raised a great deal of money for local charities and no doubt is one of the reasons that Macella was named as one of the outstanding women of Napa County in 1999.  

But Macella's true claim to fame is in the saddle where she has won in every division but specializes in developing young horses into top jumpers. In California, O’Neill has been the leading NorCal jumper rider five times, (finishing second three times, including  2009).  At the national and international level Macella has been named Leading Rider at the HITS winter desert circuit, won numerous Grand Prixs, including one in Germany and in 2009 qualified Melanie Rapp's beautiful grey, "Incandescent" to compete at Las Vegas in the National Grand Prix @ the World Cup Finals.  

This year Macella has an exceptionally exciting group of horses to ride in addition to DMS's best group ever of junior and amateur competitors. It just keeps getting better.


Charles White

Upon graduation from Calistoga High School in 1978, Charles White parlayed his accomplishments on the football field into a scholarship to Santa Rosa Junior College where his outstanding efforts earned him another scholarship to Montana State University in Bozeman.  Returning to Calistoga with O'Neill, Charlie put his business major to work and started DMS in 1983. 

Charlie has also been running the
Charles White Hay Sales out of Diamond Mountain Stable for the past three decades. He got into the business because hay sales were a natural adjunct of the horse stable.

White travels throughout northern California and up to southern Oregon many times a year on hay buying trips. 

White is also a past president of the NCHJA as well as having managed the highly successful and popular Napa Valley Classic horse show for years.  He was also the driving force behind DMS's European horse dealings, which have actually culminated in DMS being one of the very few barns that sell horses back to Europe, as well as having imported over 75 horses in one year during a busy year.  Today horses from Europe are shipped directly to DMS thanks to the volume and success of this endeavor that White initiated.

When he is not managing the hay business, White works with his partner, Macella O’Neill on the DMS horse training and show business.  White is involved in all aspects of this business.  He hauls horses to shows throughout northern and southern California, as far east as the Kentucky Horse Park and even to Spruce Meadows in Calgary, Canada.  White also coaches O’Neill at home and in the warm up arena at the shows as well as helps train the many clients who attend horse shows.

White said, “I like being involved in the animal side of the ag business.  I like being involved with people connected to the earth.”


Haley "Haha" Schwab

DMS excitedly welcomes back Haha from her 3-year riding hiatus, during which she graduated from the University of Chicago in the Phi Beta Kappa society with a degree in Geography.

Haha, an Indiana native with roots in the San Francisco Bay Area, spent her junior riding career under Charlie and Macella's tutelage. Before scampering off to higher education, Haha was a hot junior rider. She rode for the Zone 10 Young Riders team twice, earned the PCHA Grand Prix Rookie of the Year award, and began performing at the World Cup level. She headed to England for a season to work for Olympic gold-medalist Peter Charles, and began working professionally for DMS in 2013. Here she continued to foster her passion for developing young and sale horses, coaching DMS clients to success in the show ring, and riding in grand prixs. She is thrilled to be returning to her professional career as a coach and rider, and will also be an excellent addition to Charles White Hay Sales.

Isabelle Nolden


Isabelle, or “Bellini” as she is fondly known, had her first lesson at DMS when she was nine years old. She began her competitive riding career with Macella and Charlie at 16, when she moved from Marin County to St. Helena with her family. Isabelle’s athleticism, passion for horses, and love of warp-speeds brought her remarkable success in the show ring for the next four years.

After spending a number of years in Southern California, in 2015 she returned to DMS as staff. She says her job “is to do whatever needs to be done”, and her goal is to “contribute and acquire all the skills needed to run a barn, care for horses, and be a better rider." Isabelle describes her relationship with horses as "almost spiritual" but "certainly grounding". Those who work with her say she is vibrant, focused, intense, eccentric, and charismatic.

Gelacio Arriagas

Gelacio's role as the head groom is perhaps the most integral part of the DMS team and is truly the glue that holds everything together. He takes impeccable care of our horses, notices every detail, and is always available to assist when needed. For twenty years, Gelacio's meticulous work has been the cornerstone of DMS horse care. 

Claudia Colo

Of her 35 years in the horse world, Claudia says "it has been anything but boring!"  Her first 10 years were with Hap Hansen Stables.  She groomed and managed a 60 horse show barn, which competed nationally and internationally.  She also managed a dressage barn, eventing barn, and a 300 horse breeding barn.  Her passion is breeding and raising quality jumpers from birth to first grade.

As she looks back over those 35 years, Claudia recalls "my years with DMS were certainly some of my best years.   I feel grateful to be on the team again."

When she's not teaching or managing the daily barn routine, she enjoys backpacking.