The precious stones mined by Petra Diamonds Ltd (LON:PDL) lost their sparkle for Citigroup on Tuesday, although some other brokers remained partial to them.
The US broker downgraded the South African miner to 'neutral' from 'buy' with a revised target price of 120p per share.
Petra has made some notable announcements of late, including news in August of the discovery of a 138.5-carat diamond at its renowned Cullinan mine.
Citigroup said it remained positive on Petra's prospects but added that it believed the company's expected growth was already built into the price.
"The next catalysts for the stock are expected to be increased cash generation, cost savings/productivity improvements and shareholder returns, however we believe we are still a few years out on these aspects," the broker said.
However, four other brokers were more upbeat on the stock, reiterating their 'outperform', 'buy' or 'overweight' recommendations.
Canaccord Genuity said in a note that Petra's mine costs would increase this year but further weakness in the South African rand should help to offset that.
The broker's Nick Hatch said: "Diamond prices fell last year, but we expect with a better product mix and stable (or potentially improving) market prices that Petra’s average sales price may improve.
"This should enable Petra to become free cash-flow positive in fiscal 2017, a major improvement – and we expect the share price to respond accordingly as net debt starts to fall."
Macquarie repeated its 'outperform' on the miner while JP Morgan Cazenove and Barclays Capital confirmed their 'overweight' ratings.
Petra's bigger peer, Anglo American PLC (LON:AAL), was in favour with Macquarie which upgraded it to 'neutral'.
The Australian investment bank said: "We’ve increased our EBITDA forecasts for Anglo by 10-18% on CY17-18 on higher coal/ manganese price forecasts and increased our target price to 815p.
"This, coupled with our view that Anglo’s year end net debt target should be comfortably achieved, has led us to upgrade the stock to 'neutral'."