For now, they’re vacant semicircles of dark asphalt, modest extensions of the sidewalk into Boylston Street.

But in the coming months, a fusion of granite, bronze, and glass will take shape there, memorials to mark the location where two bombs detonated near the finish line of the Boston Marathon five years ago.

City officials had pledged last year that the markers of the violence, which killed three people and injured more than 260, would be unveiled this month. But the work has taken longer than expected, as the victims’ families have worked closely with the artist and city to refine the initial design.

“We were really pushing for it to be done for the fifth anniversary, but it’s more important to get it right than meet an artificial deadline,’’ said Donny Tavares, the city’s chief diversity officer, who’s overseeing the memorials. “The families wanted it to be a little more personal than the original design, and we went back and addressed their wishes.’’

Initially, city officials had envisioned a design featuring two stark obelisks, which would twist over the bombing sites and include simple sculptures at the top, representing the sacred.

The latest design, which remains a work in progress in a Chelsea foundry, is significantly more abstract and subject to interpretation. The families of the victims killed in the bombing say they like what they see so far.

With a budget of about $2 million, city officials plan to adorn the extended sidewalk areas with a combination of red bricks and different types of granite.

At the center of both sites will be a circle of dark granite, with small stone pillars that rise and twist into each other, reflecting the victims who lost their lives on April 15, 2013 — Martin Richard, an 8-year-old third-grader from Dorchester; Lingzi Lu, a 23-year-old Boston University graduate student from China; and Krystle Campbell, a 29-year-old restaurant manager from Medford.

The pillars, connected with bronze, will be a rough-hewn granite chosen by each of the victims’ families. Richard’s stone was taken from near his home in Franklin Park. Lu’s was donated by Boston University. Campbell’s is from Spectacle Island, where she worked and loved to visit.

At the site of the first bomb, where Campbell was killed, there will be one stone with two empty bronze pedestals a few inches off the ground; at the site of the second bomb, where Richard and Lu died, there will be two stones and one empty pedestal. The empty spaces are meant to link the two sites.

Outside the inner circle, there will be a second circle of a different shade of granite, representing the 16 people who lost limbs and the others who suffered wounds that day. A third, larger circle, of yet another shade of granite, will represent all those who bore witness to the horror.

Piercing the circles will be a sharp diamond of black granite, symbolizing the violence.

Along the curb at both sites, calling attention to the memorials, will be a pair of cherry trees — expected to bloom every year around Patriots Day — and four 18-foot special lights made of cast glass and enmeshed in a bronze lattice, signifying “the fragility of life and the constant cherishing that our ideals require,’’ said Pablo Eduardo, the artist who is creating the memorials.

Eduardo, a Bolivian exile who now lives in Gloucester, was chosen last year by the families of the victims, who included Sean Collier, a 27-year-old MIT police officer from Somerville, and Dennis Simmonds, a 28-year-old Boston police officer. Collier was killed three days after the attack when he was ambushed in Cambridge by the bombers, Tamerlan and Dzhokhar Tsarnaev; Simmonds died nearly a year after the attack from wounds he sustained during a gun battle with the Tsarnaevs in Watertown.

Eduardo, who studied at The School of the Museum of Fine Arts and Tufts University and has worked as a sculptor for more than 20 years, may be best known in Boston for his 10-foot bronze sculpture of Mayor Kevin White at Faneuil Hall.

Late last year, Eduardo decided to scrap his original design, which the families worried seemed too formal and impersonal.

“The design we have now is something we all came up with,’’ said Eduardo, who hopes the markers will be installed in September. “I think it’s very poetic, with the combination of all the different materials.’’

For some of the families, the previous design didn’t feel right.

“Every time we looked at it, we just didn’t see it,’’ said Bill Richard, Martin’s father. “It was beautiful, but it wasn’t right. So we went in another direction.’’

Richard thanked city officials for their patience and flexibility.

“We felt an enormous responsibility that it should stand the test of time, that it’s not too trendy, and fits the surrounding architecture,’’ he said. “The city never forced anything upon us, and we ended up in a place where everyone was comfortable.’’

Patricia Campbell, Krystle’s mom, also prefers the latest design.

“It’s absolutely beautiful — gorgeous,’’ she said. “It brings up a lot of emotions, but we’re very happy, very pleased with it.’’

The Richards, Campbells, and Lus will be given an opportunity to inscribe a message on a bronze plaque that will be fixed to the stones representing their children. The exact location of the plaques hasn’t been decided.

In the coming weeks, city officials also plan to begin seeking proposals from consultants who would oversee the construction of a larger memorial, slated for completion in 2020. The location has not been determined.

They sought such proposals last year, but the response they received convinced them to seek candidates who have experience working with trauma victims.

The consultant will eventually solicit ideas from the public, convene town hall-style meetings, and launch a digital outreach campaign. Afterward, the consultant will work with survivors to select an artist to design the memorial.

“It is of paramount importance that both the markers and memorial commemorating the victims and survivors of the Boston Marathon bombings are done in a way that is thoughtful, inclusive, and in partnership with the very best talent,’’ said Mayor Martin J. Walsh in a statement. “We will take the time that is needed to ensure that these installations are of the highest standard.’’

At Sincere Metal Works in Chelsea, where the smaller memorials will be built in the coming months, workers have been using a mix of wax, rubber, and plastic tubes to create moldings for the lights, which can be programmed to change colors.

They’ve also recently received heavy sheets of bronze, which they will use throughout the memorials.

After demonstrating how they will shape the bronze by heating it up to more than 2,000 degrees, then pouring the molten metal into ceramic casings, Dan Kendall, the owner of the foundry, described how much it means to him to help create the memorials.

“We all had friends down there that day,’’ he said. “We’ll get other big jobs, but none will carry the meaning this does.’’

David Abel can be reached at dabel@globe.com. Follow him on Twitter @davabel.