All naturally occurring uranium contains ²³⁸U and ²³⁵U (in the ratio 137.7:1). Both isotopes are the starting points for complex decay series that eventually produce stable isotopes of lead. ²³⁸U decays to ²⁰⁶Pb (half-life = 4510 Ma, see decay constant) by a process of eight alpha-decay steps and six beta-decay steps. ²³⁵U decays to ²⁰⁷Pb (half-life = 713 Ma) by a similar series of stages that involves seven alpha-decay steps and four beta-decay steps. Also included within this range of methods is that for thorium–lead dating (²³²Th to ²⁰⁷Pb; half-life = 13 900 Ma). Uranium–lead dating was applied initially to uranium minerals, e.g. uraninite and pitchblende, but as these are rather restricted in occurrence it is more normal to use the mineral zircon, even though the uranium is present only in trace amounts. The amount of radiogenic lead from all these methods must be distinguished from naturally occurring lead, and this is calculated by using the ratio with ²⁰⁴Pb, which is a stable isotope of the element then, after correcting for original lead, if the mineral has remained in a closed system, the ²³⁵U:²⁰⁷Pb and ²³⁸U:²⁰⁶Pb ages should agree. If this is the case, they are concordant and the age determined is most probably the actual age of the specimen. These ratios can be plotted to produce a curve, the Concordia curve (see concordia diagram). If the ages determined using these two methods do not agree, then they do not fall on this curve and are therefore discordant. This commonly occurs if the system has been heated or otherwise disturbed, causing a loss of some of the lead daughter atoms. Because ²⁰⁷Pb and ²⁰⁶Pb are chemically identical, they are usually lost in the same proportions. The plot of the ratios will then produce a straight line below the Concordia curve. G. W. Wetherill has shown that the two points on the Concordia curve intersected by this straight line will represent the time of initial crystallization and the time of the subsequent lead loss.