公司财务原理（Brealey and Myers）问题

请各位大牛帮忙看一下，公司财务原理（Brealey and Myers）第九章，第三节后面：什么决定资产贝塔系数

开通有这句话：

“Lone prospectors in search of gold look forward to extremely uncertain future earnings, but whether they strike it rich is not likely to depend on the performance of the market portfolio. Even if they do not find gold, they do not bear much market risk. Therefore, an investment in gold has a high standard deviation but a relatively low beta.” [Brealey and Myers]

不太明白的是，作者这句话想要做出什么假设呢，想要表达什么呀？隐隐约约度不太明白？而且，前段说风险利润等无关，我有点迷糊，麻烦各位大神帮忙看看。感谢

需要书籍的请留下邮箱，太大上传不上来。请各位帮忙指点一下。谢谢。

孤独的淘金者淘金期待极不确定的未来收益，但无论他们致富不可能依靠市场投资组合的表现。即使他们没有找到黄金，他们也不承担太多的市场风险。因此，对黄金的投资有高的标准偏差，但相对较低的贝塔值
作者其实就是用这个案例来表达贝塔的感性理解