Looking at an elevated perspective view of the drilling site the hole was drilled in apparently competent rock, not sand. It's why the walls of the hole are firm and intact. NASA personnel named the rock "paver rock".
The photograph posted in the question was taken at night,
in order to reduce self-shadowing within the borehole that can occur during daylight imaging.
This close up night time shot indicates the hole and the drill cuttings have a green-gray hue, which could indicate the presence of olivine - a green colored magnesium iron silicate of igneous origin. As discussed in the question, Difference between “mafic floor” and “olivine-bearing floor” in Jezero crater? (Perseverance landing site)
I am assuming the coring bit was used to drill the hole, which would also indicate why the wall of the hole can sometimes be jagged, it depends on the rock being drilled and its condition: oxidized, fresh, stressed or not.
The rover's drill will use rotary motion with or without percussion to penetrate into the Martian surface to collect the precious samples. The drill is equipped with three different kinds of attachments (bits) that facilitate sample acquisition and surface analysis. The coring and regolith bits are used to collect Martian samples directly into a clean sample collection tube, while the abrader bit is used to scrape off or "abrade" the top layers of rocks, to expose fresh, un-weathered surfaces for study.
The "lump" in the central base of the hole could either be clump of drill cutting that fell into the hole, or it could be the remnant stub from which the core sample was snap off. The breakage points from where core samples are taken are rarely smooth.
From the picture below, it looks like the location of drilled hole was the second attempt at drilling the hole. There is ground disturbance to the left (at approximately 10 o'clock to the hole) of the successful hole which looks like an abandoned attempt at drilling a hole - a shallow "circular" gray depression and grey dust surrounding the depression.
Edit - A few hours later, following a comment, 14 August 2021
Apart from the quality of the light in the photograph in the question, one reason for the sides of the hole appearing like the drill cuttings is the way the hole has been drilled has most likely left a thin coating of drilling dust on the sides of the hole.
On Earth, diamond core drilling usually involves flushing the bit with water during the drilling process. This would ensure any drill cuttings would not clog the bit resulting in the bit becoming jammed in the hole. This would be critical for long holes.
If air flushing is used this will result in a coating dust being smeared along the hole. This is what happens during rotary air blast (RAB) drilling where compressed air is forced through the central hollow of the drill steel, exists through the holes in the bit and is forced to exist the hole via the gap between the outside of the drill steel and the sides of the hole. Most of the expelled dust/cutting exit the hole, some of the dust is smeared along the sides of the hole. RAB holes do not produce core, just drill cuttings/chips.
Perseverance would not have used water to flush the hole. Drilling relatively short holes, it may not have flushed the hole, which would still have resulted in dust coating the sides of the hole.
Edit - 14 August 2021
NASA's analysis of the this, first hole, states the "commanded" drilling depth of 7 cm was achieved, but despite initial data suggesting core had been recovered from the hole, sealed and stored, no core was recovered. The conclusion was,
the coring activity in this unusual rock resulted only in powder/small fragments which were not retained due to their size and the lack of any significant chunk of a core. It appears that the rock was not robust enough to produce a core. Some material is visible in the bottom of the hole. The material from the desired core is likely either in the bottom of the hole, in the cuttings pile, or some combination of both. We are unable to distinguish further given the measurement uncertainties.
The assessment that the rock was not robust enough, unusual and that it only produced powder/fragments indicates the rock was friable/crumbly. This better explains the appearance of the hole cuttings and the texture of the sides of the hole.
It would also indicate the rock may be weathered which would not be surprising, given that Jezero Crater is thought to have been flooded with water.
Further Edit - same day
Another source claims NASA scientist think the rock drilled was basalt.
NASA scientists reckoned the rock was either a sedimentary or a basalt (crystalized magma). Given how the rock behaved when drilled, now they are leaning towards basalt, which crystallizes at depth to form coarse grains. “When we started to core this rock, it basically broke up along these kind of disintegrating grain boundaries,”
... igneous rock like basalt provides a timeline: Scientists can date when the magma turned into hard rock.