Apple diehards who keep saying the stock "will come back" are going into hibernation.

Shares of the gadget maker closed down another 2.7 per cent Friday to $US106.03 - knocking the stock down 21 per cent from its recent high of $US134.54. The breathtaking decline not only puts Apple into a bear market - defined by a 20 per cent drop - but has obliterated a staggering $US160 billion ($223 billion) in shareholder wealth from the top.

Apple's Tim Cook is resisting a push by the FBI to unlock the iPhone of the San Bernardino shooter. Credit:AP

Just to put that into perspective, Apple's $US160 billion decline is larger than 477 companies in the Standard & Poor's 500 are worth. A drop this big is the financial equivalent of wiping out the market value of entire companies like Pepsico at $US146 billion, IBM at $US133 billion or Nike at $US111 billion.

Seeing such a massive decline in Apple carries more weight than a similar decline in any other stock would. Apple is still worth more than any other US company - making it the most important stock in market measures like the S&P 500. Apple also is the most widely held stock by individual investors, says Sigfig, so the decline directly hits home.