Many UFO reports seem to focus on objects described by witnesses as either shaped like a diamond, saucer, or disc, or as very bright -- almost blinding -- lights.
On the evening of April 5, 2012, over the Costa Mesa area of California, a motorist reportedly saw a diamond-shaped object hovering about a mile away from him.
In his report filed with the Mutual UFO Network, or MUFON, he also claims to have heard a humming sound emanating from the object.
"I saw what appeared to be a diamond-shaped vehicle in the eastern sky approximately one mile from my location," reported the witness, whose identity is being withheld while the case is under investigation.
"While I was driving, I noticed the vehicle hovering, so I thought it was a helicopter, but it did not have aviation lights, and it was much too big to be a helicopter."
When the witness pulled off to the side of the road for a better look, he said the object tilted on its side.
"And then I was able to see it was in the shape of actually two diamonds connected in the middle," he said in the report. "At first I thought it was a hoax, but this vehicle was much too sophisticated to be a toy, and very large. It made a humming noise that I could hear from approximately a mile away."
The witness added that the UFO had four lights on it that he described as "brighter than the sun lights."
Writing about this case in The Examiner, newly appointed MUFON director of public relations Roger Marsh noted that there were no images or videos included with the original report filed on April 6.
"The diamond shape is probably in the top six or seven most reported UFOs," Marsh told The Huffington Post.
"What's different about this case is the fact that the witness on the ground heard a humming sound -- in most good UFO cases, the object is completely silent," Marsh said.
Marsh, who is also editor of the MUFON UFO Journal, said he knows a little about quiet UFOs from his own personal close encounter experience.
In 1973, Marsh and about a dozen other eyewitnesses near Greensburg, Pa., say they saw an unusual site in the sky.
"There was this disc-shaped object hovering just above a telephone pole. It was perfectly silent, completely lit up on the bottom, the width of a two-lane road, and it went right over the top of us," Marsh recalled.
After that object performed a variety of up and down and hovering maneuvers in front of Marsh and other onlookers, they say it took off in the sky.
"Within a couple minutes, suddenly there were Air Force jets circling the area and eventually went off in the direction of where the UFO had gone."
Despite this close experience with an unexplained object under his belt, Marsh said he believes the more recent diamond-shaped California UFO could still turn out to have an earthly origin.
"I would have to say that if something is a mile away, I'm believing the witness is reporting what he saw, but the percentage that it could be something manmade increases with the distance," he said.
Marsh added that March UFO sightings, filed with MUFON, included many shapes reported by eyewitnesses around the country.
"The sphere remains the most-reported UFO shape with 169 March reports," he said. "Other shape reports include: star-like, 113; circle, 107; triangle, 96; disc, 84; oval, 68; unknown, 68; other, 67; fireball, 51; square/rectangular, 31; cylinder, 31; flash, 30; cigar, 21; boomerang, 21; diamond, 19; egg, 13; bullet-missile, 12; chevron, 9; Saturn-like, 8; cone, 6; cross, 5; teardrop, 5; and blimp, 2."
Watch this 2009 video of a reported diamond-shaped UFO over Pasadena, Calif.: